docs(objectui): concept.mdx collapsed-by-default section example authors collapsible too (#19838)

Fixes #19703
Clause-②: no

## What changed

`content/docs/protocol/objectui/concept.mdx`, section "1. Progressive
Disclosure": the YAML example captioned "Advanced section (collapsed by
default)" authored `collapsed: true` with no `collapsible`. It now
writes both keys (`collapsible: true` + `collapsed: true`), and one
sentence under the block states the rule the spec states: `collapsed:
true` implies `collapsible` — a section that starts closed always
carries the disclosure control that reopens it. +4 / -0, one file, docs
only.

## Why

The spec half is already on `main`: `packages/spec/src/ui/view.zod.ts`
(FormSectionSchema, "The collapse pair, declared once for both members")
declares that `collapsed` IMPLIES `collapsible` as a renderer rule
(maintainer ruling 2026-09-18, letter A), and that parse never rewrites
the pair. So a section authored `collapsed: true` alone parses to
`collapsible: false, collapsed: true`, and any renderer that reads the
parsed `collapsible` without applying the implication draws a section
that starts closed with no toggle. Writing both keys is correct under
the spec's wording (`collapsible: true` beside `collapsed: true` is the
fully explicit spelling) and renders a working toggle on every surface.

Measured against the built spec at `a3091327`:
`FormSectionSchema.safeParse` of the corrected section returns success
with `{collapsible: true, collapsed: true}`; the old section returns
`{collapsible: false, collapsed: true}` — the premise holds.

## Renderer surfaces (objectui at the `.objectui-sha` pin `main` carries
today, `87af769e9a3e`)

The pin has not moved since the card was filed, so the card's citation
still applies. Read at that pin:

- `packages/plugin-form/src/DrawerForm.tsx:609` and `:677` pass
`collapsible: section.collapsible` alone (and `:653` gates collapse on
`section.collapsible`). The old example rendered closed with no toggle
here; with both keys it renders a toggle and starts closed. **Safe with
the corrected example.**
- `packages/plugin-form/src/ObjectForm.tsx:1539-1560` applies the
implication (`collapsed` implies `collapsible`). Safe with either
spelling; **safe with the corrected example.**

Writing both keys does not depend on the pin: it is safe whether or not
a later pin makes DrawerForm apply the implication.

## Changeset

Docs-only: `content/docs/**` is not in any package's `files[]`, so
nothing publishes — `skip-changeset` territory. Labels are left to the
PM seat (no label write from this PR's author).

## Verification

Derived with `node scripts/pm/dispatch-gates.mjs --repo
objectstack-ai/objectstack --commands` (change set taken from git, merge
base `1cacfe4a`): 40 commands, all run at `a3091327`, all exit 0.
Reconciled with `--ran`: "40 derived famil(ies) accounted for — 40 run,
0 NOT-MEASURED (a DERIVED zero — all 40 recorded an exit code and none
of them is 3)". Four gates first exited 3 (prerequisite not met:
`@objectstack/lint`, `@objectstack/formula`, `@objectstack/client-react`
unbuilt); after building those closures they re-ran green.

Note: `check:yaml-examples` validates only tagged blocks and reports
`concept.mdx: 0 tagged / 9 untagged`, so its green does not cover this
block; the direct `FormSectionSchema.safeParse` reading above is the
measurement for it.

## Acceptance notes

- Sibling pages, silences rather than copy-and-fail, **not edited
here**: `content/docs/ui/views.mdx:416-417` (section key table lists
`collapsible` / `collapsed` with no word on the dependency) and
`content/docs/protocol/objectui/layout-dsl.mdx` (the same pair at
several lines; its authored example already writes both keys). Carrier:
none named.
- The DrawerForm residual (reads `section.collapsible` alone) is
objectui's, not this repo's.

---
_Generated by [Claude
Code](https://claude.ai/code/session_01VDtqoecgES7ScQYGbFVDRv)_

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

Author: objectstack-fleet[bot]

content/docs/protocol/objectui/concept.mdx

@@ -593,10 +593,14 @@ layout:
       fields: [name, email, phone]
     - name: advanced_settings
       label: Advanced Settings
+      collapsible: true
       collapsed: true
       fields: [api_key, webhook_url, rate_limit]
 ```
 
+`collapsed: true` implies `collapsible` — a section that starts closed always carries the disclosure
+control that reopens it — and the example writes both keys so that control is explicit on every form surface.
+
 ### 2. Contextual Actions
 
 Actions appear based on state. `visible` and `disabled` are **CEL predicate strings**