[composite] Change focus to sync

[atomiks]

Fixes #828

This removes Composite's extra focus microtask now that floating focus return is already async, and preserves radio group selection behavior when arrow navigation moves focus synchronously.

Changes

• Focus the next composite item synchronously during keyboard navigation.
• Keep radio group arrow navigation selecting the focused radio in the same event turn.
• Remove obsolete flushMicrotasks() noise from the Composite root tests and add direct coverage for synchronous focus.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@4484
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@4484
  

commit: 1f204fb

[mui-bot]

Bundle size report

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+262B(+0.06%)	🔺+54B(+0.04%)

Details of bundle changes

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