fix(desktop): keep context gauge stable at the transcript tail

[me2seeks]

Summary

Keep the composer context gauge visually stable when the transcript reaches the live tail.

The tail transition fades the scroll-to-bottom control inside the same filtered dock surface as the composer. Chromium can re-raster that shared surface even though the gauge, model-control group, and composer DOM boxes never move; the thin gauge paths then visibly hop by a device pixel. This patch adds paint containment and a compositing hint to the scroll button’s existing 32px transition container, limiting the opacity repaint to that control.

The existing reader-scroll Storybook scenario now includes a real context gauge and asserts both sides of the contract: the scroll-button paint boundary is present, and the gauge SVG stays within 0.25px while returning to the tail.

Fixes #4973

Verification

• npm --workspace @maka/desktop run typecheck:stories
• node --test packages/ui/dist/__tests__/composer-context-usage.test.js (2/2)
• npm --workspace @maka/desktop run build-storybook
• node scripts/storybook-visual-smoke.mjs apps/desktop/storybook-static (312 stories, 339 theme renders)
• npm run check:renderer-architecture -- --base origin/main (101/101 fixtures; check passed)
• npm run astryx:surface-inventory
• npm run lint
• npm run format:check
• git diff --check

AI use

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool and scope: Codex — diagnosis, implementation, browser verification, and test coverage.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck, and affected suites pass locally

Does this PR entail a change in behavior?

• Yes — the context gauge no longer visually jumps during the tail transition
• No