fix(ui): draw the reasoning chevron from the Astryx icon registry

[Astro-Han]

Summary

The disclosure chevron on the Thinking row rendered with a visibly heavier stroke than the identical chevron on tool rows.

Root cause: two different chevron sources. Tool rows use Astryx <Icon icon="chevronDown" size="xsm"> — a 24-viewBox glyph carrying the theme's stroke-width: 1.75. ChatReasoning, ejected from the Astryx lab package, hand-wrote its own chevron at 12 viewBox / strokeWidth 1.5. Both are then forced to 10x10 by chat-message.css, so the effective stroke was 1.25px vs 0.73px — the same-looking glyph, ~70% heavier on one row. Path geometry was otherwise identical.

The reasoning chevron now goes through the registry (<Icon icon="chevronDown" size="xsm" color="inherit" />), so the app has one chevron authority and one place to retune it. The registry's string mode wraps its svg in an astryx-icon span, so the 10x10 rule now covers the wrapper (> *) on both rows, not just on tool rows — the two selectors collapse into one :is() pair.

The file header already documents the product-dialect deviations layered on the ejected lab DOM (cursor, hover wash, chevron size); this deviation is recorded there too.

Verification

Measured in Storybook (Product/Shell Official AppShell → NativeConversation) against the live DOM, not eyeballed — getComputedStyle(path).strokeWidth × path.getScreenCTM().a for the trailing chevron of each disclosure:

	viewBox	stroke (user units)	CTM scale	effective stroke
before — reasoning	0 0 12 12	1.5	0.8333	1.25px
before — tool calls	0 0 24 24	1.75	0.4167	0.7292px
after — reasoning	0 0 24 24	1.75	0.4167	0.7292px
after — tool calls	0 0 24 24	1.75	0.4167	0.7292px

Layout and motion are unchanged: both chevron wrappers still measure 14x14 with a 10x10 svg, margin-inline-start: auto intact, and expanding still rotates the chevron 180°.

(One measuring trap worth recording: read the expanded chevron's transform in a throttled background tab and you get an identity matrix, because the CSSTransition sits at currentTime: 0 forever. Force a paint first, or getAnimations().forEach(a => a.finish()).)

Also run:

• npm run format / npm run format:check / npm run lint — clean
• npm run typecheck (all workspaces) — clean
• node --test "packages/ui/dist/**/*.test.js" — 342 pass, 0 fail, including the new chevron contract
• apps/desktop main tests — the 12 failures in project-root-controller / project-management reproduce on a clean main checkout (tmpdir symlink resolution) and are untouched by this change
• Not run: Playwright E2E — no user journey changes.

Two contracts guard the invariant, one per half, each verified to fail on the state it claims to catch:

• processing-block.test.tsx slices the turn at the two row roots and compares each row's own chevron markup. Restoring the pre-fix component fails on "the reasoning row must draw exactly one registry icon"; swapping the row to chevronRight fails on "both rows must render the same chevron markup".
• chat-disclosure-chevron-contract.test.ts (new, at the existing css-test-helpers seam) asserts each row declares both its chevron svg and its wrapper 10x10. Dropping the reasoning arm from the :is() fails; dropping the wrapper arm fails. It pins the outcome, not the wording — resplitting or reordering the selector list stays green.