[Singmin]

Timeline layer entries are clickable list items, so keyboard users cannot select a layer to inspect its events. Their Enable/Disable buttons are focusable but remain transparent without mouse hover.

Render layer selection as a native button beside the existing collection button, preserving the list structure without nested buttons. Expose the selected layer with aria-pressed, include the layer name in collection-action labels, and reveal collection controls on focus within the row. Add inset keyboard focus outlines, keep disabled-collection dimming off the focus indicators, and reserve space so long layer names do not overlap the action.

Add three integration regressions using the actual Timeline state logic for built-in and dynamic layer semantics, focusability, repeated selection, and independent collection controls. All three fail on the original page and pass with the fix.

Relates to #885. This is limited to the left-hand layer list; #1170 covers Timeline event rows. A local three-way merge with #1170 at 1b0e2e8 is clean.

Validation

• pnpm exec vitest run --maxWorkers=4: 68 files, 408 tests passed.
• Targeted regression tests, pnpm typecheck, pnpm check, and git diff --check: passed.
• Full package build: passed. On Windows, ran the two steps of pnpm build separately to preserve the quoted filters:
  • pnpm --filter './packages/*' --filter '!vite-plugin-vue-devtools' -r build
  • pnpm --filter vite-plugin-vue-devtools build
• Local Chrome 155 fixture with the actual Timeline, Splitpanes, UnoCSS, and RecycleScroller: Tab/Shift+Tab order, Enter/Space activation, repeated selection, collection-action isolation, visible focus in both themes, long names, and all 48 layer controls in a narrow layout passed. With 1,000 events per event layer, 48 event views were mounted; 28px event rows, scrolling, and mouse selection passed with no page errors.

Integration tests stub layout and scrolling; native key activation and layout were checked separately in the browser fixture. Full host smoke suites and screen-reader testing were not run.

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