[combobox] Return highlight to selected item on query clear

[atomiks]

Closes #5227

Supersedes #5148.

Clearing a query replaces the filtered item tree while the selected item must be highlighted and scrolled back into view. The old registry committed refs, values, labels, and indexes separately, so a Combobox-only retry could still navigate using partially updated data.

Composite now publishes one settled, index-addressed item snapshot. This also covers #5148: explicitly indexed non-virtualized items remain registered for pointer navigation, lifecycle resets repopulate the element registry, and explicit and automatic indexes can safely coexist by reserving explicit slots. These belong together because both features require the same atomic registry invariant.

Changes

• Restore the selected item after manual, closing, and selection-triggered query clears.
• Support controlled indexes, DOM reorders, virtualization, and closed-trigger typeahead through the same committed registry.

Measurements

Measurement	Result vs. master
Open 500 items	-10.8% React / -8.9% paint
Filter, 500 remain	-7.5%
Filter, ~11 remain	+0.17% filter work; 17 → 15 renders

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@5232
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@5232
  

commit: f1a8060

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+2.06KB(+0.46%)	🔺+774B(+0.53%)

Details of bundle changes

Performance

Total duration: 1,187.46 ms -183.18 ms^(-13.4%) | Renders: 76 ^(▼-2) | Paint: 1,875.98 ms -247.25 ms^(-11.6%)

Test	Duration	Renders
Slider mount (300 instances)	113.09 ms ▼-52.36 ms(-31.6%)	2 (▼-1)
Tabs mount (200 instances)	180.82 ms ▼-48.26 ms(-21.1%)	3 (▼-1)
Scroll Area mount (300 instances)	77.14 ms ▼-28.10 ms(-26.7%)	3 (+0)

11 tests within noise — details

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