#29076 base red: webviews check:browser-lanes, check:design-tokens, check:type-scale, vp lint

Summary

The feat-cmux-next tip failed four webviews checks (seen by merge-batch at a8615fdaedc8, still red at 25c15849ff9). The pre-push hook refused every squash that touches webviews. This PR fixes each finding in the code. No checker, baseline or allowlist changes.

Check	Finding at 25c15849ff9	Culprit	Fix
check:browser-lanes	test/composer-draft-overflow.test.ts:434 samples in a raw rAF loop	dc0fd32ab02 (Austin Wang, #28672 work, direct push)	The sampler reads through window.cmuxAfterPaint, which open already installs
check:design-tokens	ContextSection.css padding 2px 6px 0	fbea666a0f0 (#28969)	4px 8px 0; the link's own 2px side padding is taken back (margin-right: -2px) so its text stays on the rows' 8px trailing edge
check:type-scale	pages/apps/styles.css: 8 raw sizes, baseline 7 (new .apps-instance 12px)	d13d17acfaf (direct push)	var(--text-detail) (12px) with its letter spacing
vp lint	SectionList.tsx:73 jsx-a11y(prefer-tag-over-role) on role="group"	6b7c28f69f9 (direct push)	Reasoned disable: the suggested tags (fieldset, details, optgroup) are for form controls or disclosure, not a labelled run of nav links (same as DrillList)

vp fmt --check is already green at this tip (#28927).

Changelog

none

Observed / Not run

• Observed: bun run check in webviews/ at base 25c15849ff9: 4 red: check:browser-lanes, check:design-tokens, check:type-scale, vp lint. At head 95efc8b71df2f421da9240aeba1ed743593ba8f6 (rebased on a0d51f836ae): exit 0.
• Observed: scripts/web-lint.sh passed; prepush-gate.py: no new violations.
• Observed: CMUX_BROWSER_TESTS=1 bun test test/composer-draft-overflow.test.ts -t "never under a button|inset comes back": 5 pass, 0 fail (Chromium).
• Not run: the WebKit lane of that test (WebKit is not installed on this Linux VM). No visual capture of the 2px change in the Context section's Compact row (the text position is the same; the row is 2px taller at the top).

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Summary by cubic

Fixes four red webviews checks on the feat-cmux-next tip so the pre-push hook no longer blocks webviews squashes.

• check:browser-lanes: the draft-overflow sampler now reads through window.cmuxAfterPaint instead of a hand-rolled rAF + MessageChannel loop.
• check:design-tokens: the Context section's Compact row padding moves from 2px 6px 0 to 4px 8px 0; the link's own 2px side padding is offset so its text keeps the rows' 8px trailing edge.
• check:type-scale: .apps-instance uses var(--text-detail) with its letter spacing instead of a raw 12px.
• vp lint: SectionList gets a reasoned disable for role="group" since the suggested tags (fieldset, details, optgroup) target form controls or disclosure, not navigation links.