[combobox] Support primitive item values

[atomiks]

Alternative to #4686.

Partially addresses #3594. Related to #4419 and #4735.

This lets Combobox use primitive values with supported object items, such as items={[{ value: "banana", label: "Banana" }]} with defaultValue="banana" or <Combobox.Item value="banana" />.

The public API stays the same: Combobox infers primitive values from supported item shapes instead of adding an itemValue prop. Select is unchanged in this PR.

Unlike #4686, this keeps the existing index and value tracking (selectedIndex in the store, allValuesRef) instead of reworking it, so the change stays small and contained.

Changes

• Support primitive Combobox values for { value, label } items.
• Use inferred values for selection, highlight, scroll into view, typeahead, autofill, and labels, while preserving the exact rendered value shape for <Combobox.Item value={item}>.
• Keep Combobox item values available while filtering, closing, using inline lists, or rendering without items.
• Add a value-shape test matrix across item types (primitive, primitive key, whole { value, label } item, arbitrary object with itemToStringLabel/itemToStringValue, and object-valued labeled item) covering selection, highlight, labels, custom equality, autofill, and virtualized cases.

Notes

• This only infers values for the supported { value, label } item shape.
• Other object item shapes still need matching values or isItemEqualToValue.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@4985
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@4985
  

commit: c05db74

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[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+1.18KB(+0.25%)	🔺+351B(+0.24%)

Details of bundle changes

Performance

Total duration: 1,153.94 ms -220.06 ms^(-16.0%) | Renders: 50 ⁽⁺⁰⁾ | Paint: 1,745.84 ms -329.15 ms^(-15.9%)

Test	Duration	Renders
Tabs mount (200 instances)	215.69 ms ▼-55.53 ms(-20.5%)	4 (+0)
Menu mount (300 instances)	130.19 ms ▼-34.16 ms(-20.8%)	2 (+0)
Menu open (500 items)	62.66 ms ▼-26.58 ms(-29.8%)	12 (+0)

9 tests within noise — details

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