Merge commit 'c91cc055c8ad81afd8f055b71c74af6819020637' into feat/felo6-group-parents

# Conflicts:
#	context/test-evidence/formula-and-io.md

context/code/cli.md

@@ -369,3 +369,26 @@ non-zero and writes nothing. It is checked before the build rather than read
 off the build's error, so the listing is structured data and not a parsed
 sentence. `separate` uses `ImportPipeline::suggested_duplicate_category`, and
 there is deliberately no flag to name the category yet.
+
+## 12 — `import` takes folders, and the CLI only says which arguments are folders (improvise-lpzu)
+
+`improvise import` takes any mix of files and folders. A folder stands for its
+`*.csv` files, in name order. A folder always merges, even when it holds a
+single CSV, so that file keeps its `File` item and view. A single file named
+directly still imports alone (the user's ruling on the PR). The rules
+and their reasons are in `context/code/formula-and-io.md`, "A folder of CSVs is
+one sparse model, with a view per file". The CLI's part is the one question
+`Storage` cannot answer: whether a path is a folder. `get_import_data` asks
+`Path::is_dir` for each argument, hands the resulting `ImportPath`s to
+`expand_import_paths` over `PhysicalFs`, and from there takes the path a list
+of files always took. A folder with no CSV in it is refused before anything is
+read. The message names the folder, and the run exits non-zero and writes
+nothing.
+
+A merge now reaches the build as an `ImportPipeline` carrying the merged files'
+`sources`, rather than as a bare JSON value. Both `--no-wizard` and the TUI
+wizard (`run_tui` → `App::start_import`) therefore build the per-file views.
+The headless path matters most, because the code-maat pipeline
+(improvise-1xfb.5.1) runs there. `a_headless_import_of_a_folder_merges_its_csv_files_and_repeats_byte_for_byte`
+pins the views, the sparse Data lines and the byte-identical second run, with
+the first run's output sitting in the folder it reads.

context/code/formula-and-io.md

@@ -658,6 +658,144 @@ one proves nothing.
 backend rather than only a file (improvise-fuj); the path-taking wrappers remain
 for the native CLI.
 
+### A folder of CSVs is one sparse model, with a view per file (improvise-lpzu)
+
+`improvise import <folder>` exists for improvise-1xfb.5.1. code-maat writes one
+CSV per analysis into a directory, and the schemas differ: coupling has
+`entity,coupled,degree,average-revs`, age has `entity,age-months`, and
+abs-churn has `date,added,deleted,commits`. The user ruled (2026-09-25):
+"Records are already sparse: the model can just leave null/empty values for
+columns not in a given CSV file." So files with different headers merge into
+**one** model. It is not refused, and it is not split into one model per schema.
+
+**A folder argument means exactly its `*.csv` files.** `expand_import_paths`
+takes the arguments in order and replaces each folder with its `*.csv`
+entries. The extension check is case-insensitive, the listing is not recursive,
+and the order is `Storage::list`'s, which is sorted by contract. Then the
+existing path runs, with one difference: **a folder always merges**. Two or more
+files merge through `merge_csvs_from` with a `File` category, and so does a
+folder holding a single CSV. That gives the one file its `File` item and its
+per-file view.
+
+The single-CSV rule is **the user's ruling (2026-09-25)**, which replaced this
+PR's first answer, "`import dir/` is exactly `import dir/*.csv`". Their words:
+"yes, we might want to have some kind of update import eventually (don't plan
+this yet, but we'd want file identity for this)". A folder names a collection
+that may grow, so a document built from it should record which file each
+record came from from the start.
+
+**A single CSV named directly, not through a folder, still imports alone,
+without `File`.** That is deliberate. Naming one file asks for that file, not
+for a collection, and the shipped recipes (`examples/demo.improv` from
+`examples/demo.csv`) and every existing single-file script would otherwise gain
+a `File` category and a second view. File identity for update-import can be
+asked for by naming a folder. The test is
+`a_folder_of_one_csv_still_gets_a_file_category_and_its_view_but_a_lone_file_does_not`.
+
+Apart from that, a folder still behaves as the glob would, and four cases follow
+from that rather than being chosen one by one:
+
+- **Non-CSV entries are skipped**, not refused, because a glob skips them. This
+  includes the output `.improv`, which a second run finds in the folder.
+- **A folder with no CSV at all is refused**, naming the folder. A glob that
+  matches nothing is an error too, and importing nothing would write an empty
+  document.
+- **Files and folders mix freely**, in argument order.
+- **A file named twice is read twice.** For example, `dir/a.csv dir/` reads
+  `a.csv` twice, and its records collide with themselves. The headless path
+  refuses that like any other collision. Nothing deduplicates, because the
+  shell glob would not either.
+
+The listing goes through `Storage::list` so the browser host can reuse it
+(gotcha 2). `Storage` knows documents, not folders, so **whether an argument
+is a folder is the caller's question**. The CLI answers it at the edge with
+`Path::is_dir` and passes an `ImportPath::{File, Folder}`. An OPFS host would
+answer it from its own UI.
+
+**Sparse merge.** A record from a file without some column has no key for it,
+so `Plan::place` gives it no coordinate there (improvise-8jk6.2 above), and no
+measure cell. Nothing is fabricated: no `0` and no `""`. The analyzer already
+classified each column over the values present (improvise-l1f5), so a column
+only one file has is judged by that file's values alone. The category cap
+counts the **union** of every file's categories, header-only files included
+(see the next paragraph). Rows from different files cannot collide, because
+`File` differs. One caveat on "judged by its values": a JSON `""` is not yet
+dropped by the analyzer the way `null` is, so a JSON numeric column holding
+`""` is proposed as a Label (improvise-8jk6.11). CSV is unaffected, because
+its parser turns a blank field into `null`.
+
+**A header-only file is still a file** (the Codex P2 on PR #314). code-maat
+writes a header and no rows whenever nothing passes its threshold, so
+`coupling.csv` is often exactly that. The pipeline is analyzed from *records*,
+and such a file has none, so it had no `File` item, its columns never reached
+the model, and its view filtered to an item that did not exist.
+`ImportPipeline::from_merge` therefore **seeds from `Merged::sources`**:
+
+- the `File` proposal's items are the sources' items, in file order, so every
+  file has its item whether it has rows or not;
+- a source column that no record has a value in is proposed as a **Label**.
+  That is the analyzer's own verdict for an all-null column (improvise-l1f5),
+  and a column with no values is exactly that: nothing can be classified, so it
+  takes no pivot slot and does not count toward the category cap until the user
+  reclassifies it in the wizard or with `--category`/`--measure`.
+
+This is also the file identity the user wants kept for a future update-import.
+
+**A file with no measure column gets a records view** (improvise-8jk6.13). A
+record with no measure makes no cell, so such a file contributes items and no
+data. Its view had nothing to page `_Measure` to, and paged onto another file's
+measure, which the same view hides; the cell there was a `Record`, editable,
+under this file's `File`. Now `source_view` gives such a file the records
+layout (`view::records_steps`, the one home of what a records view is) with
+`File` still on Filter at the file's item. It draws zero rows, because there
+are no cells. A header-only file whose columns are
+all Labels is the same case. In records mode the columns are the whole model's
+categories and measures, not just this file's: records mode has no per-view
+column filter. With no rows that is cosmetic.
+
+**Each file gets a view** (the user, 2026-09-25: "dir import should do this:
+one view filtered by the file column and projected to the columns in that
+file"). `merge_csvs_from` returns a `Merged` whose `sources` list each `File`
+item with the columns its header named. Two files with one stem share an item,
+so their columns are unioned. The pipeline carries `sources`, and the build
+adds one view per source after the merged `Default` view, which stays the
+initial view. Each per-file view:
+
+- **Is named after the file's stem.** A name already taken, including the
+  virtual `_Records` and `_Drill`, gets ` 2`, ` 3` and so on. A view named
+  `_Drill` would never be written and would be dropped on load.
+- **Puts `File` on `Axis::Filter`, fixed at that file's item.** Filter and
+  not Page, because the view *is* that file's projection. Paging to another
+  file would show that file's records through this file's columns and hidden
+  measures, which is wrong on every page but one. Filter constrains without
+  being navigable (gotcha 27). Until now it appeared only in the never-saved
+  `_Drill` view, but the `.improv` grammar has always round-tripped it.
+  `per_file_views_survive_a_round_trip_and_repeat_byte_for_byte` proves that on
+  a saved per-file view, and `a_saved_per_file_view_projects_only_its_file`
+  proves the shared `grid_in` projection, which both the TUI and the browser
+  draw from, carries it as a fixed coordinate with nothing to cycle.
+- **Seats the file's own categories** (its columns, plus date parts derived
+  from them) through `View::on_category_added`, in model order: the rule a
+  single-file import uses. Every other category goes on `None`. The file's
+  records lack those, so nothing splits.
+- **Puts `_Measure` on Page**, as `Workbook::new` does, **with other files'
+  measures hidden** and the page selection on the file's first measure. A
+  formula target comes from no file, so it stays visible. A file with no measure
+  gets a records view instead (above).
+- **Is never named like another view from the same import.** `Default.csv` and
+  `Default 2.csv` become `Default 2` and `Default 2 2`; without the same-batch
+  check both would be named `Default 2` and one view would silently replace the
+  other.
+
+A per-file view is made only while `File` is an accepted category. A user who
+skipped or reclassified it has no filter to build views from.
+
+Two things this does not do. Page cycling still steps onto a hidden item and
+draws an empty grid: `[`/`]` in a per-file view can reach another file's
+measure (improvise-yd2t; the Default view already behaved this way). And a
+CSV with its own `File` column has it overwritten by the merge's, which is the
+existing behaviour of `merge_csvs`.
+
 ### The wizard's back path is a history stack
 
 `advance` pushes the step it leaves; `previous` pops it. Keeping the path
@@ -684,10 +822,35 @@ and a blank string are **absences, not names**; stringifying them used to invent
 an item literally called "null" (improvise-ych). Every other value keeps its
 printed form.
 
-A record missing a value for an accepted Category field has no place in the grid
-and is dropped; `build_model_with_warnings` counts it against the *first*
-category it is missing and reports the total, because without a warning that
-loss is invisible in a thousand-row import (improvise-5j9).
+**A record with no value for a category keeps its cell, without that
+coordinate** (improvise-8jk6.2). Records are sparse: a `CellKey` names only the
+coordinates it has. So a missing field, a `null` and a blank string all mean
+"no coordinate here". None of them makes an item: not `"null"`, not `""`, and
+no synthetic placeholder. The same rule covers labels, which used to become an
+item named `""`. The user's words, 2026-09-25: "this limit in import should be
+fixed, it dates to when this wasn't sparse." That limit was improvise-5j9's
+rule: such a record "has no place in the grid" and was dropped, counted and
+reported as `Skipped N of M records missing category values`. 5j9's warning
+made the loss visible, and the loss is now gone, so the warning went with it.
+The only record still skipped is one that is not a JSON object.
+
+Where such a record shows up is decided by the axes, not by the import. The
+test is `a_record_with_no_coordinate_is_drawn_only_where_that_category_is_off_the_grid`:
+
+- **The category on Row or Column:** no position holds the record. Its other
+  coordinates may still draw a row, but the cell under the category's items is
+  empty, and the column total leaves it out.
+- **The category on Page:** no selection shows it.
+- **The category on None:** it is aggregated like any other record.
+- **Records mode:** its row shows a blank cell in that category's column.
+- **`.improv`:** its Data line omits the category, and it round-trips.
+- **Drill:** a drill that does not constrain the category reaches it.
+
+**No "(none)" position, by the user's decision (2026-09-25).** They were asked
+whether Row, Column or Page should give such records a place of their own. Their
+answer: "I think leaving it out for now is fine, maybe a UI element somewhere
+that shows the number of records underneath the current view?" That indicator
+is improvise-gtn7.10, and it is not an import concern.
 
 ### Colliding records are a question, not a default (improvise-cgqt)
 
@@ -704,7 +867,9 @@ made it choose too. It chose `Refuse` and points at `:import`.
 
 **Detection comes before the build and shares the build's placement.** Where a
 record lands is `Plan::place`: its categories, then its date parts, then its
-labels, then its measure values, or a reason it is dropped. `collisions()` and
+labels, then its measure values. Only a record that is not an object is
+dropped, since a missing value is no coordinate rather than a reason to drop
+(improvise-8jk6.2). `collisions()` and
 `build_model_with_warnings` both call it, so the question the wizard asks and
 the thing the build does cannot disagree. Neither has a second walk of the
 record. **A collision is a cell that more than one record writes**, meaning a

context/test-evidence/cli.md

@@ -407,3 +407,54 @@ named, and no output file. It has two siblings. `..._sums_duplicates_when_told_t
 asserts `= 30` and no `Occurrence`. `..._keeps_duplicates_separate_when_told_to`
 asserts the category, `= 10` and `= 20`, and no `= 30`. So the two flags cannot
 both be summing.
+
+### a_headless_import_of_a_folder_merges_its_csv_files_and_repeats_byte_for_byte
+
+improvise-lpzu, at the process boundary. The folder holds code-maat's three
+schemas (coupling, age and abs-churn) and a `README.txt`. At `e65d87fd`'s
+parent the run failed with `Cannot read '…/maat': Is a directory (os error
+21)`, because a single path went straight to `read_to_string`. The test
+asserts:
+
+- sparse Data lines for all three schemas, with no README;
+- the four views, `Default` first and initial, then one per file;
+- `File: filter, coupling`;
+- a second run, with the first run's `maat.improv` now inside the folder,
+  writing the same bytes.
+
+Rivals R2, R5, R6 and R7 each fail it.
+
+### a_headless_import_takes_files_and_folders_together_and_refuses_an_empty_folder
+
+improvise-lpzu. A file argument and a folder argument together are one merge,
+and the extra file's records carry its own `File` item. Before the fix this
+failed with `Multi-file import only supports CSV files`, because the folder was
+not a `.csv` path. An empty folder exits non-zero, writes nothing, and names
+itself: `no CSV files in folder …/empty`. Rivals R2, R4 and R5 each fail it.
+
+### a_saved_per_file_view_projects_only_its_file
+
+improvise-lpzu (`tests/folder_import_views.rs`). A folder import is saved,
+reloaded and projected through `project::grid_in`, the one derivation the TUI
+grid and the browser both draw from (gotcha 44). This is why no separate TUI
+or browser test was written. For each file's view:
+
+- the page bar carries `File` fixed at that file with nothing to cycle
+  (`items: None`, gotcha 27);
+- the drawn cells are that file's alone: `2.00, 7.00` for age and
+  `5.00, 3.00` for coupling.
+
+Rivals R2, R7 and R9 each fail it. Since the review of `4e68372c` the fixture
+also holds a header-only `soc.csv`. Its view must carry `File` at `soc`, be a
+records view, and draw zero rows. Rivals F7 and F8 fail that half.
+
+### a_folder_of_one_csv_still_gets_a_file_category_and_its_view_but_a_lone_file_does_not
+
+improvise-lpzu, after the user's ruling on PR #314 ("we'd want file identity").
+At `4e68372c`, a folder holding one CSV took the single-file path, because
+`import dir/` was exactly `dir/*.csv`. That document had no `File` category and
+no per-file view, and the test failed at the `## Category: File` assertion. The
+other half pins the direction the ruling did *not* change: the same CSV named
+directly imports without any `File`. The rival that merges every all-CSV
+import, lone files included, fails that half ("a lone file keeps today's
+shape"). So the test separates "a folder merges" from "everything merges".

context/test-evidence/formula-and-io.md

@@ -1026,6 +1026,14 @@ every blank column value in a CSV import grew a phantom "null" item. Null means
 *absent*: the record must be skipped and counted, exactly as if the key were
 missing.
 
+**The skip is superseded by improvise-8jk6.2.** Null still means absent, and
+still invents no item. But an absent coordinate no longer drops the record: it
+keeps its cell without that coordinate. So the test now asserts both cells
+(`region=East` and the bare `_Measure=revenue`) and no warning. The rival that
+restores the skip (a present-but-blank category returns the record as
+unplaceable) fails here, beside
+`a_record_with_no_value_for_a_category_is_kept_without_that_coordinate`.
+
 ### build_model_literal_string_null_is_a_real_category_item
 
 The other side of improvise-ych: the *string* `"null"` is an ordinary value — a
@@ -1038,7 +1046,9 @@ null" would pass the test above.
 improvise-ych, third arm. An empty string is the JSON spelling of a blank cell —
 the CSV parser already turns a blank field into `Value::Null`, so the two inputs
 must agree. An item with an empty name is unnameable in the grid, so it is
-treated as missing rather than created.
+treated as missing rather than created. Since improvise-8jk6.2 "missing"
+means the record keeps its cell without a `region` coordinate. It is no longer
+skipped, and the same skip-restoring rival fails here.
 
 ### build_model_sums_records_that_share_every_coordinate
 
@@ -1107,12 +1117,19 @@ and scrolls them. It was red at compile time against the old `Option<String>`
 signature, then red against a `Vec` that still held one elided sentence. The
 rival **elided after three** fails it.
 
-### build_model_warns_about_records_skipped_for_missing_category
+### improvise-5j9's skip warning, retired by improvise-8jk6.2
 
 improvise-5j9: skipping a record for a missing category value used to be silent,
-so a 1000-row import could drop rows with no user-visible sign. The warning must
-name the count, the total, and the offending field(s) — a bare "some records were
-skipped" is not actionable.
+so a 1000-row import could drop rows with no user-visible sign. The warning had
+to name the count, the total, and the offending field(s); a bare "some records
+were skipped" is not actionable. Its test,
+`build_model_warns_about_records_skipped_for_missing_category`, was **deleted**
+by improvise-8jk6.2. A record missing a category value is now kept without that
+coordinate, so there is no skip left to report. Its successor is
+`a_record_with_no_value_for_a_category_is_kept_without_that_coordinate`, which
+asserts no warning at all. `build_model_record_with_missing_category_value_skipped`
+became `build_model_record_with_missing_category_field_is_kept` and now asserts
+the kept cell rather than only the East one.
 
 ### is_does_not_swallow_the_start_of_a_longer_word
 
@@ -1436,7 +1453,16 @@ The fix reports and does not relocate: the row still lands with the coordinates
 it does have, exactly as before. Changing where such a row is placed is a
 behaviour change the ticket did not ask for.
 
-### import::wizard::tests::a_row_dropped_for_a_missing_category_is_not_charged_to_the_date_tally
+### import::wizard::tests::a_row_kept_without_a_category_is_charged_to_the_date_tally
+
+**Renamed from `a_row_dropped_for_a_missing_category_is_not_charged_to_the_date_tally`
+by improvise-8jk6.2.** Its premise went with the skip. A row missing `Region`
+now lands, so its unparseable `Date` *is* a derived column's loss, and it is
+charged once to each of `Date_Month` and `Date_Year`. The test asserts the
+landed cell (`Date=2025-02-05, _Measure=Amount = 200`, with no `Region` and no
+derived part) and exactly those two warnings. The history below is kept
+because the rule it established (charge only landed rows) still holds. Every
+object row now lands.
 
 improvise-yss5, found by ChatGPT Codex reviewing PR #150 at `af7fae0e` and
 correct. The first fix incremented `unparsed_dates` while walking a record's
@@ -1659,3 +1685,184 @@ The fix makes the state unrepresentable (`Expanded(NonZeroUsize)`), so the
 regression guard is the type, and the old failing input no longer compiles.
 This test covers every fold the API CAN build (`Collapsed` and depths 1 to 3,
 beside a category depth), each saved and read back.
+
+### a_record_with_no_value_for_a_category_is_kept_without_that_coordinate
+
+improvise-8jk6.2, reported by the user from the wizard: "Skipped 6 of 342
+records missing category values: Settlement Date (6)". `Plan::place` returned
+`Placement::Missing` for a category with no value, so the build dropped the
+whole record, measures and all. The fixture has one complete record, one with
+`"Settlement Date": null` and one with `""`, for two different payees. At
+`e65d87fd`'s parent the build stored only the first cell. Now the stored cells
+are exactly the three records, the two blanks with no date coordinate, and
+there is no warning. `Settlement Date` holds only its one real item, so neither
+`"null"` nor `""` became one. Rival R1, a record with a present-but-blank
+category treated as unplaceable, fails this test and the two improvise-ych
+tests.
+
+### files_with_different_headers_import_as_one_sparse_model
+
+improvise-lpzu. code-maat's `age.csv` (`entity,age-months`) and `coupling.csv`
+(`entity,coupled,degree,average-revs`) merged through `merge_csvs_from`. Before
+the fix, the age records had no `coupled` key, so they took the same `Missing`
+path and **every age record was dropped**. The build stored the four coupling
+cells and nothing else. The test pins the proposals (each column's kind,
+judged over its present values), all six stored cells sorted, no empty-named
+item in any category, and no warning. Rival R2, a record whose file lacks a
+category column skipped, fails it along with the two CLI folder tests, the
+projection test and the per-file view test.
+
+### a_label_column_a_record_lacks_is_no_coordinate_not_an_empty_item
+
+improvise-lpzu, the label half. Labels were attached with
+`scalar_name(..).unwrap_or_default()`, so an age record, whose file has no
+`coupled`, stored `coupled=` as a coordinate, and `coupled` grew an item named
+`""`. Before the fix the first sorted cell was
+`File=age, _Measure=age-months, coupled=, entity=src/a.rs = 2`. Rival R3
+(`unwrap_or_default` restored) fails it.
+
+### a_folder_expands_to_its_csv_files_in_name_order_and_skips_the_rest
+
+improvise-lpzu. The folder holds three CSVs written out of name order, one
+with an upper-case `.CSV`, plus a `.txt`, an `.improv` (the output a second run
+finds) and a CSV in *another* folder. Only the three are returned, sorted.
+`MemoryFs` stores in a `HashMap`, so the order comes from `Storage::list`'s
+contract and not from insertion. Rival R5 (the extension filter dropped) and
+rival R6 (the listing passed through a `HashSet`) each fail it.
+
+### a_folder_with_no_csv_files_is_refused_by_name
+
+improvise-lpzu. Both an empty folder and a folder holding only non-CSV files
+must refuse with `no CSV files in folder maat`. Importing nothing would write
+an empty document. Rival R4 (the `ensure!` removed) fails it, and so does R5,
+because the text-only folder then "has" files.
+
+### files_and_folders_expand_in_argument_order
+
+improvise-lpzu. `[z.csv, b/, a.csv]` must expand to
+`[z.csv, b/one.csv, b/two.csv, a.csv]`. Argument order is kept, and a folder
+is sorted within itself. A fixture listing only folders, or files in name
+order, could not tell "argument order" from "sort everything". R6 fails it.
+
+### two_files_with_one_stem_union_their_columns
+
+improvise-8jk6.12 (3). `a/x.csv` (`entity,one`) and `b/x.csv` (`entity,two`) are
+one `File` item, `x`, whose columns are `entity, one, two`. Rival F3 (the
+reviewer's fault: clear the stem's columns before each file) leaves
+`entity, two`, and the `x` view would then hide the measure `one` that half its
+records carry.
+
+### files_with_different_headers_merge_without_inventing_the_missing_columns
+
+improvise-lpzu, at the parser. Each merged record carries exactly its own
+file's keys plus `File`: no key, `null` or `0` for a column another file has.
+Each `SourceFile` lists its file's columns in header order. The per-file views
+are built from those lists.
+
+### a_record_with_no_coordinate_is_drawn_only_where_that_category_is_off_the_grid
+
+improvise-8jk6.2's downstream question, asked of the real layout rather than
+argued. Bolt has no `Date`:
+
+- With `Date` on Column or on Page, the grid draws only Acme's `10`.
+- With `Date` on None, it draws both `10` and `20`.
+- In records mode, Bolt's row reads `["", "Bolt", "", "20", ""]`, a blank
+  `Date` cell.
+- The Data line is `Payee=Bolt, _Measure=Amount = 20`, and it round-trips.
+- A drill on `Payee=Bolt` reaches the record.
+
+This is a characterization of the model's existing sparse semantics. It found
+no bug. Its purpose is to make the Row/Column answer ("invisible") a stated
+fact the user can overrule, rather than an accident. R2 fails it.
+
+### a_merge_gives_each_file_a_view_projected_to_its_columns
+
+improvise-lpzu, the user's "one view filtered by the file column and
+projected to the columns in that file". The test asserts:
+
+- the view order (`Default`, then the files in read order), with `Default`
+  still active;
+- `File` on Filter at the file's item;
+- the file's own categories seated, and the other file's on None;
+- `_Measure` on Page at the file's first measure, with the other file's
+  measures hidden;
+- the drawn values.
+
+The age view draws `[2, 7]`. The coupling view draws `[5, 3]`, because its
+first measure in model order is `average-revs`: serde_json's map orders keys
+alphabetically. Failed at `bb505f8f`'s parent, where no view existed. Rivals
+R7 (File on None), R9 (every category seated) and R10 (nothing hidden) each
+fail it.
+
+### a_files_view_holds_only_that_files_records
+
+improvise-lpzu. The code-maat fixture cannot show that the *filter* works,
+because its files share no measure, so the measure page alone would isolate
+them. Two quarterly files with the same `Region,Revenue` schema can: without
+the filter, `q1` would draw `30` and `3`. Rival R7 fails it. R9 and R10 do
+not, and are not meant to.
+
+### a_files_view_name_skips_names_already_held
+
+improvise-lpzu. Two stems collide with names already held: `Default`, the
+merged view, and `_Drill`, a virtual view that is never written and is dropped
+on load. The first becomes `Default 2` and the second `_Drill 2`, and each
+still filters to its own item. Rival R8 (the virtual names not checked) fails
+it: the file's view would have been saved as `_Drill` and lost.
+
+### per_file_views_survive_a_round_trip_and_repeat_byte_for_byte
+
+improvise-lpzu. Two builds from two independently written `MemoryFs` give
+identical `.improv` text, and `format(parse(text)) == text`. The reloaded
+document still has the three views in order, `File: filter` at the file's item,
+and the same drawn values. This is the first persisted, non-virtual use of
+`Axis::Filter` (gotcha 27). R7 and R9 fail it.
+
+### a_header_only_file_keeps_its_file_item_its_columns_and_its_view
+
+The ChatGPT Codex P2 on PR #314 at `4e68372c`, confirmed by the adversarial
+review. `age.csv` (one row), a header-only `coupling.csv` and `rev.csv` (one
+row). At `005b7a76` the test failed at the first assertion: `File` held only
+`age` and `rev`, so the `coupling` view filtered to an item that did not exist,
+and `coupled`, `degree` and `average-revs` were not in the model. Now `File`
+reads `age, coupling, rev` in file order, the three columns exist (as Labels),
+and the `coupling` view is a zero-row records view filtered to `coupling`, while
+the `age` view still draws `2`. Rival F5 (sources not seeded) and F6 (unseen
+columns not proposed) each fail it, and so do F7 and F8 below.
+
+### a_file_with_no_measure_gets_a_records_view_not_another_files_measure
+
+improvise-8jk6.13, from the review of PR #314. `a.csv` (`entity,x`) and
+`tags.csv` (`entity,tag`). At `005b7a76` the `tags` view had `_Measure` on Page
+with no selection, so the layout paged onto `x`, a measure the same view hides.
+The test failed there, with `row_count() == 1`: `a`'s record, drawn under
+`File=tags`'s view. Now the view has no `_Measure` page, is a records layout,
+and draws zero rows, while `a`'s view is still a pivot. Rival F7 (the old
+paging) fails it, and F8 (the File selection lost when `records_steps` moves
+`File` off Filter) fails it with the same `1`. F8 is the mistake the first
+version of the fix made.
+
+### a_files_view_seats_the_date_parts_of_its_own_date_column
+
+improvise-8jk6.12 (1). The reviewer replaced the date-parts clause of
+`source_view`'s `own` with `false` and every test passed, because no folder
+fixture extracted a date part. Here `abs-churn.csv` has a `date` column,
+extracted to `date_Month`, and `age.csv` does not. `date_Month` is seated in the
+`abs-churn` view and on None in the `age` view. Rival F1 (the reviewer's fault)
+puts it on None in both and fails the first assertion.
+
+### two_files_whose_names_collide_in_one_batch_keep_a_view_each
+
+improvise-8jk6.12 (2). `Default.csv` and `Default 2.csv`. The first takes
+`Default 2`, so the second must take `Default 2 2`. Without the same-batch
+check both are named `Default 2`, and `IndexMap::insert` silently replaces the
+first, leaving two views where three are owed. The test asserts all three names
+with their filters. Rival F2 (the reviewer's fault) fails it.
+
+### a_whitespace_only_category_value_is_no_coordinate
+
+improvise-8jk6.12 (4). A `region` of `"   "` is kept as a record with no
+`region` coordinate, and `region` holds only `East`. The trim in `scalar_name`
+predates this PR (improvise-ych), but the sparse-record rule now rests on it:
+without it the value becomes an item named `"   "`. Rival F4 (the reviewer's
+fault) fails it.