[checkbox] Fix parent group cancellation and indeterminate state

[atomiks]

This fixes Checkbox and CheckboxGroup edge cases where internal handling could continue after a canceled change, or manually indeterminate parent checkboxes could expose the wrong state.

Changes

• Let onCheckedChange cancellation run before CheckboxGroup parent/child handling.
• Keep parent-enabled group state unchanged when a parent or child change is canceled.
• Apply manual parent indeterminate state to ARIA and the hidden input state.
• Tighten parent-group tests so child checkbox assertions do not silently skip the default span root.

Original findings addressed

• [P1] Parent checkboxes could be visually mixed while exposing aria-checked="false".
• [P2] eventDetails.cancel() was too late for parent-enabled CheckboxGroup changes.
• [P2] Parent-group tests had no-op child assertions.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@4941
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@4941
  

commit: 5356871

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+39B(+0.01%)	🔺+19B(+0.01%)

Details of bundle changes

Performance

Total duration: 1,131.04 ms -205.69 ms^(-15.4%) | Renders: 50 ⁽⁺⁰⁾ | Paint: 1,701.59 ms -331.97 ms^(-16.3%)

Test	Duration	Renders
Slider mount (300 instances)	156.55 ms ▼-64.03 ms(-29.0%)	3 (+0)
Checkbox mount (500 instances)	61.16 ms ▼-25.90 ms(-29.8%)	1 (+0)

10 tests within noise — details

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Check out the code infra dashboard for more information about this PR.

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