[radio group] Fix disabled selected form submission

[atomiks]

Radio groups now match native form submission when the selected radio is disabled, and the indicator state type reflects the root state it already receives.

Changes

• Return null from form values when the tracked selected radio is disabled.
• Add Chromium coverage for radios disabled after mount and initially disabled.
• Expose the full radio root state through Radio.Indicator.State docs and types.

Original findings addressed

• [P1] Disabled selected radios were excluded from native FormData but still reported by onFormSubmit.
• [P2] RadioIndicatorState only declared checked even though it receives root field state at runtime.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@4926
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@4926
  

commit: 3b7512f

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+68B(+0.01%)	🔺+17B(+0.01%)

Details of bundle changes

Performance

Total duration: 1,325.67 ms -19.90 ms^(-1.5%) | Renders: 50 ⁽⁺⁰⁾ | Paint: 2,000.82 ms -60.92 ms^(-3.0%)

Test	Duration	Renders
Checkbox mount (500 instances)	106.99 ms 🔺+25.58 ms(+31.4%)	1 (+0)

11 tests within noise — details

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