fix(spec): step 18's rationale gains the form-view-subform-columns-closed fragment (#21090)

Part of #20901

Clause-②: no

One `STEP18_RATIONALE` fragment in
`packages/spec/src/migrations/registry.ts`: id
`form-view-subform-columns-closed`, `order` 54 (the highest at base
`5dbeb7d7b7` was 53), inserted where its id sorts. It covers the
form-view carrier closure, the D2 conversion
`form-view-subform-columns-canonicalized`, and the identity-only sibling
`inline-grid-column-identity-only-currency-scale-refused`. Each sentence
restates the two entry sources and the conversion source. The list sits
outside the generated markers and is hand-written;
`gen:migration-registry` is a no-op on this diff. A `patch` changeset
for `@objectstack/spec` is included because the rationale string ships
in `dist/migrations`. This is review item 4 of the at-tier record
`5921114874` on #20953. The card stays open, and the seat closes it by
hand.

## Verification at head `8ebaf237c4` (merges origin/main `dff98c1f51`)

- Rationale pins (`step18-rationale-merge`,
`compliance-families-retirement`, `conversions-major18-merge`): 31
passed. `migrations.test.ts` and `inline-grid-column-carriers.test.ts`:
179 passed. Spec local suite: 588 files, 17318 passed. Spec typecheck:
exit 0.
- `check:generated`: all 15 artifacts are up to date after a spec build.
The upgrade guide prints majors up to `PROTOCOL_MAJOR` (17), so it does
not print step 18 and has no diff here.
- `dispatch-gates --ran`: 82 derived, 82 run, 0 NOT-MEASURED.
- Ablation (fragment removed, anchor 1 to 0, restored to the HEAD blob):
no pin went red. The merge test pins the sort order and the joined text;
no test pins this fragment by id.

## Acceptance notes

- The review counted entries without a fragment of their own id: 207 of
253 at `06f079864a`. Fragments are per retirement family, so the number
of families without one is not counted. `carrier:` none named. Noted
here under the filing gate, with no `reach:` measured today. The
generated guide prints major 17 only (`build-upgrade-guide.ts` loops to
`PROTOCOL_MAJOR`, 17 at this head). Step 18's rationale does reach `os
migrate meta --step`, whose default chain ends at major 18
(`packages/cli/src/commands/migrate/meta.ts`). There, a family without a
fragment is left out of the paragraph, not misstated. This was read from
the source and not measured at that door.

---
_Generated by [Claude
Code](https://claude.ai/code/session_01Sfe5YjBLwB9J3y8fvm2xq1)_

Co-authored-by: Claude <noreply@anthropic.com>

Author: objectstack-fleet[bot]

.changeset/20901-step18-rationale-form-view-subform-columns.md

@@ -0,0 +1,10 @@
+---
+'@objectstack/spec': patch
+---
+
+fix(spec): protocol 18's migration rationale now covers the form view's inline grid columns and the identity-only currency `scale` refusal (#20901)
+
+**`@objectstack/spec`**
+
+- **`MIGRATIONS_BY_MAJOR[18].rationale` gains one fragment, `form-view-subform-columns-closed`.** It is the paragraph `os migrate meta --step` shows for the protocol 17 → 18 hop. The new sentences say that a form view's `subforms[].columns` now takes the strict `InlineGridColumnSchema` a relationship field's `inlineColumns` takes, that the conversion `form-view-subform-columns-canonicalized` respells a `{ field }` column as `{ name }` in stored rows and assembled artifacts while an author writing `field` is refused, and that `defineStack` refuses `scale` on a column that declares no `type` when its `name` is a `currency` field of a child object declared in the same stack (`inline-grid-column-identity-only-currency-scale-refused`).
+- Text only: no schema, conversion or migration entry changes, and `conversionIds` and `semantic` for step 18 are unchanged.

packages/spec/src/migrations/registry.ts

@@ -5504,6 +5504,37 @@ const STEP18_RATIONALE: readonly RationaleFragment[] = [
       + 'the key with the prescription; the mechanical conversion strips it from stored '
       + 'sources (pure lossless delete — it never had an effect on this surface to lose).',
   },
+  {
+    id: 'form-view-subform-columns-closed',
+    order: 54,
+    text:
+      'It also closes the form view\'s inline grid columns: `subforms[].columns`, on `view.form` '
+      + 'and on `formViews` entries, was `z.array(z.any())` while a relationship field\'s '
+      + '`inlineColumns` was already the strict `InlineGridColumnSchema`, so a mis-keyed column '
+      + 'published clean and drew a blank grid column, and `scale` on a currency column, which the '
+      + 'other carrier refuses under the maintainer\'s rulings of 2026-09-23 (option B) and '
+      + '2026-09-24 (option 乙), published green. The carrier now references that schema, so both '
+      + 'carriers are judged by it, with its own prescriptions. The D2 conversion '
+      + '`form-view-subform-columns-canonicalized` respells a `{ field }` column as `{ name }`, the '
+      + 'respelling `field-column-lists-canonicalized` makes on `inlineColumns`: it rewrites stored '
+      + 'rows and assembled artifacts and lists the edit under `os migrate meta`, and it is retired '
+      + 'from the load path, so an author writing `field` meets the refusal. A view saved with a '
+      + 'failing column is refused with the column schema\'s prescription, and a stored row carrying '
+      + 'one is diagnosed at rehydration; neither is stripped, because which column an unknown key '
+      + 'or a mixed `field`/`name` entry meant is the author\'s call, and a conversion that dropped '
+      + 'the key would accept at load what the parse now refuses. Its D3 record is the semantic '
+      + 'entry `form-view-subform-columns-closed`. On both carriers, the reach of '
+      + '`inline-grid-column-currency-scale-refused` extends to a column that declares no `type`: '
+      + 'such a column takes its type from the child field, which the column schema cannot see, '
+      + 'when the console hydrates it, so `defineStack`\'s cross-reference check re-parses a column '
+      + 'whose `name` is a `currency` field of the child object as the type it renders as, and the '
+      + 'refusal of its `scale` is the column schema\'s own. Reach: the child object must be '
+      + 'declared in the same stack; a column naming no field of it, or a subform whose child '
+      + 'object comes from another package, is not judged there. It has no D2 conversion, for the '
+      + 'declared-type entry\'s reason: deleting the key is the migration, and a conversion that '
+      + 'dropped it would accept it at load, the grace window ruling B refused. Its D3 record is '
+      + 'the semantic entry `inline-grid-column-identity-only-currency-scale-refused`.',
+  },
   {
     id: 'list-view-page-mount-retired',
     order: 27,