docs(adr): virtual categories and measures are defined by criteria, never declared (improvise-cq1q.1)

Their ids and displays are hardcoded; an old file's _Measure names are
recognised, not migrated as declarations. Open question 2 narrows to
where a new-version file declares the stored measures.

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

context/adr/007-stable-identities.md

@@ -19,7 +19,7 @@ amendment is approved:
 | clause | accepted rule | the amendment proposes |
 |---|---|---|
 | 1 | groups have no id | a group has an id and a display, and its id behaves like every other id |
-| 1 | `_index`, `_dim`, `_measure` are the only ids beginning with `_` | a leading `_` is reserved for virtual ids in general, including the virtual views `_records` and `_drill`; a file never declares a virtual category and refers to one by its id (`_measure`) |
+| 1 | `_index`, `_dim`, `_measure` are the only ids beginning with `_` | a leading `_` is reserved for virtual ids in general, including the virtual views `_records` and `_drill`; virtual categories and measures are defined by criteria with hardcoded ids and displays, are never declared in a file, and are referred to by id (`_measure`) |
 | 2 | `CellKey` pairs are `(CategoryId, ItemId)` | `(CategoryId, ItemHandle)` |
 | 1 | (silent; today `measure_item_names` merges a formula into a stored measure of the same name) | a formula never merges: it gets its own id, `profit-<n>` beside a stored `profit` |
 | 2, 5 | a collapsed entry and a group reference name a group by name | by its id |
@@ -102,13 +102,18 @@ the reservation needs no check in the grammar.
 - **[Amendment] Virtual ids are lower case.** The code, the wire, `ViewState` and the
   browser client spell the virtual categories `_Index`, `_Dim` and `_Measure` today,
   and move to the lower-case ids in the re-key step (clause 11).
-- **[Amendment] A file never declares a virtual category; it refers to one by its
-  id.** The virtual categories exist in every document, so they are not required in
-  the file. Where a line refers to one, it spells the virtual id:
-  `_measure: column`, `_index: none`, `region=east, _measure=revenue = 1200`. An
-  old-version file's `_Measure` resolves through its display at load, like every name
-  in an old file (clause 5), and the first save writes `_measure`. Where the file
-  declares the stored measures themselves (`Revenue`, `Cost`) is open question 2.
+- **[Amendment] Virtual categories and virtual measures are defined by criteria, not
+  declared.** Each contains whatever in the document meets its criterion: `_measure`
+  holds the measures, and `_index` and `_dim` hold the record positions. Their ids and
+  displays are hardcoded in the application, so a file never declares one. Where a
+  line refers to one, it spells the virtual id: `_measure: column`, `_index: none`,
+  `region=east, _measure=revenue = 1200`.
+- **[Amendment] An old file's virtual names are recognised, not migrated as
+  declarations.** The loader recognises `_Measure`, `_Index` and `_Dim` by their
+  hardcoded displays, and an old `## Category: _Measure` section is read for the
+  stored measures it lists, not as a declaration of `_measure`. The first save writes
+  `_measure` wherever the file refers to it. Where a new-version file declares the
+  stored measures themselves (`Revenue`, `Cost`) is open question 2.
 - **[Amendment] Records mode's synthetic positions are values, not ids.** The row
   coordinate under `_index` is a value of that virtual category. So, by the same
   reasoning, is the column coordinate under `_dim` (a column index, gotcha 1). They are
@@ -803,29 +808,30 @@ which contradicts clause 1.
 
 ### 2. Where does a file declare the stored measures?
 
-**The question.** A file never declares a virtual category (clause 1). But the stored
-measures, such as `Revenue` and `Cost`, are items of `_measure`, and each needs a
-declaration that carries its display and id. Today they are listed under
-`## Category: _Measure`:
+**The question.** `_measure` is never declared: it holds whatever meets its criterion,
+and its id and display are hardcoded (clause 1). But the stored measures in it, such as
+`Revenue` and `Cost`, are ordinary things with a user's display and a generated id, so
+the file must declare each one. Today they are listed under the virtual category's own
+header, which a new-version file no longer writes:
 
 ```
 ## Category: _Measure
 - Revenue, Cost
 ```
 
-**Why it matters.** Without a declaration a measure has no display or id in the file,
-and a new-version file refuses any undeclared id (clause 5).
+**Why it matters.** Without a declaration a stored measure has no display or id in the
+file, and a new-version file refuses any undeclared id (clause 5). The declaration is
+also what puts a stored measure into `_measure`'s criterion.
 
 **Options.**
-- **(a) Keep a section for the virtual category's items only**, spelled by its id:
-  `## Category: _measure` with `- Revenue(revenue), Cost(cost)`. The section declares
-  items, not the category.
-- **(b) A dedicated section**, such as `## Measures`, with the same item lines.
-- **(c) Declare a measure where it is first used**, in a data key. A data key holds
-  only an id, so the display would be lost.
+- **(a) A dedicated section**, such as `## Measures`, with item lines
+  (`- Revenue(revenue), Cost(cost)`). Being declared there is what makes something a
+  stored measure.
+- **(b) A marker on an ordinary item line**, wherever the measure is listed, for
+  example `- Revenue(revenue) [measure]`.
 
-**Author's recommendation:** (b). It names what the section holds, and keeps the
-virtual category out of the file entirely.
+**Author's recommendation:** (a). It names what the section holds, and keeps the
+virtual category's header out of the file.
 
 ### 3. Does interactive typing still resolve a display name that matches no id?