[combobox] Fix autofill and selected state edge cases

[atomiks]

Part of the Codex sweep. Ports the Select autofill and selected-state fixes (#4934) to Combobox, with the review findings from that port folded in.

Changes

• Browser autofill matches an item's rendered label, not just its serialized value, so a primitive value="US">United States autofills correctly. Works for inline children and the items prop.
• Field state (dirty, validation, error clearing) runs through the shared useValueChanged effects and respects onValueChange/onInputValueChange cancellation. A canceled autofill, selection, or input change no longer touches field state, and a canceled single selection keeps the popup open.
• Multiple mode clears data-dirty when the value returns to its initial set, comparing element-by-element with the item comparator instead of by array reference.
• When the input is inside the popup, the trigger's aria-controls points at the dialog popup (a stable ${rootId}-popup id shared by the popup and trigger) instead of the listbox.
• Combobox.Label resolves to Combobox.Label.State in the generated docs.

Original findings addressed

• Canceling a single-select item press still closed the popup; it now stays open, with a test.
• The ${rootId}-popup id was built in both the popup and the trigger; it's now one getComboboxPopupId helper, with the custom-id path covered.
• The autofill forceMounted mount was sticky and ran for multiple and none modes that never match; it's now scoped to single-selection autofill.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@4972
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@4972
  

commit: 8b0a579

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+457B(+0.10%)	🔺+151B(+0.10%)

Details of bundle changes

Performance

Total duration: 1,263.23 ms -152.66 ms^(-10.8%) | Renders: 50 ⁽⁺⁰⁾ | Paint: 1,893.10 ms -243.08 ms^(-11.4%)

Test	Duration	Renders
Mixed surface mount (app-like density)	86.11 ms 🔺+16.99 ms(+24.6%)	5 (+0)
Tabs mount (200 instances)	228.38 ms ▼-75.38 ms(-24.8%)	4 (+0)
Slider mount (300 instances)	153.35 ms ▼-55.12 ms(-26.4%)	3 (+0)
Checkbox mount (500 instances)	61.73 ms ▼-27.99 ms(-31.2%)	1 (+0)
Popover mount (300 instances)	64.03 ms ▼-20.45 ms(-24.2%)	1 (+0)

7 tests within noise — details

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