test+docs: derive #304's cap fixtures from MAX_CATEGORIES; record the coupling found (improvise-felo.1)

The duplicate step's cap check already reads the one constant after the
merge; its tests filled 12 categories and now fill to the cap. An
injected literal-12 check fails keeping_duplicates_separate_is_refused_at_the_category_cap.
core.md section 9 gains the inventory of what degrades as categories grow.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

context/code/core.md

@@ -575,6 +575,66 @@ the outlier, and width is not its cause: it grows 4× when the **records**
 double, at either width. `RecordRow::group` finds each cell's row with a linear
 scan (improvise-sk32).
 
+#### Coupling found
+
+Raising the constant was used to look for code that assumes few categories.
+This inventory was taken at felo.1's merge of `gh/main` `4f38a35e`. Line
+numbers are dated, so re-derive them before citing.
+
+**Reads the constant (fixed in felo.1).** The wizard's precondition, and the
+duplicate step's cap check that improvise-cgqt added (`separate_blocked`). A
+fixture that fills to the cap either derives it or fills until
+`add_category` refuses.
+
+**Separate limits, left alone.** None of these is a category count:
+
+- `MAX_ROW_HEADER_W = 24` (`improvise-protocol/src/render.rs`) is the widest
+  one header level is drawn, in cells. It equals the new cap by coincidence.
+- `MAX_FEATURES = 64` (`improvise-gp/src/fit.rs`) is the encoded-width budget
+  of one fit.
+- `CATEGORY_THRESHOLD = 20` (`import/analyzer.rs`) is the distinct-value count
+  at which a text field becomes a `Label`.
+
+**Degrades as categories grow.** These are not fixed here; each is a place
+where 24 is worse than 12. Most were already reachable at 12.
+
+- **The TUI row header is the sum of its levels.** In `src/ui/grid.rs`, around
+  `row_header_width`/`data_area_width` (and
+  `ui::viewport::compute_row_header_width`), each level takes 4 to 24 cells.
+  Categories on Row therefore cost at least 4 cells each, and the data area
+  saturates to zero width with no data drawn.
+- **Column headers take one line per level, but the keypress path does not
+  count them.** `App`'s `visible_rows` is `term_height − 8`, a fixed allowance
+  that ignores the column levels, the group row and the page bar. So with
+  several categories on Column, the cursor can scroll below the drawn grid.
+- **The page bar is one unclipped line.** It is `GridWidget::render`'s
+  `page_coords` join. Default seating (`View::on_category_added`) sends every
+  regular category after the second to Page. A 24-category import therefore
+  puts 22 selectors on one line, clips most of them without a mark, and opens
+  on a single slice.
+- **Side panels do not scroll.** In `src/ui/panel.rs`, an item past
+  `item_height` is not drawn, and the cursor can move onto it unseen. The
+  category panel lists every category plus its expanded items.
+- **The wizard's field list does not scroll.** Its `ReviewProposals` arm in
+  `src/ui/import_wizard_ui.rs` breaks at the popup's bottom (the popup is at
+  most 30 rows high), unlike the steps that use `scrolling`. About 22 fields
+  fit, so the over-the-cap case cannot show every field it asks the user to
+  fix.
+- **Loading the store is O(k log k) per cell and O(cells × k) in memory.** It
+  measured 3× from 12 to 24 (the table above).
+- **A fit declines more columns.** `unaffordable_categories` drops the widest
+  category columns once the encoding passes `MAX_FEATURES`. A wide document
+  is fit on fewer of its categories, and the fit says so.
+- **`cross_product` is exponential in the categories placed on one axis**, and
+  `prune_empty`'s `has_value` matrix is rows × columns. The cap never bounded
+  either.
+- **Header repeat suppression costs O(levels) per position and level**
+  (`render.rs::shows`). That is O(rows × levels²) per frame, which is small at
+  24.
+
+The browser adapts where the TUI does not. `#tiles` and `#categories` scroll,
+and `web/shell.js` charges each column level to the extent.
+
 ### Seating takes the kind, because a name cannot carry it (improvise-cj4u)
 
 `View::on_category_added` takes a `CategoryKind` beside the name. It used to