[navigation menu] Fix interaction, value, and styling-hook bugs

[atomiks]

This is part of the Codex sweep for Navigation Menu. It restores the local fixes from the recovered branch while preserving newer upstream changes.

Changes

• Prevented disabled triggers from opening through hover, click, or keyboard paths.
• Avoided duplicate onValueChange calls when switching items via keyboard.
• Treated non-nullish falsy item values as valid open values.
• Exposed the documented actionsRef.unmount() action.
• Pinned left-side popup transitions to the correct edge.
• Documented the missing Navigation Menu styling hooks.

Original findings addressed

• Disabled triggers could still activate their content.
• Keyboard activation could emit duplicate value changes.
• Falsy item values were handled like closed values in root cleanup.
• The documented imperative unmount action was not wired up.
• Popup and data-attribute styling hooks were incomplete.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@4942
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@4942
  

commit: ffefc22

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+45B(+0.01%)	🔺+30B(+0.02%)

Details of bundle changes

Performance

Total duration: 1,128.39 ms -208.34 ms^(-15.6%) | Renders: 50 ⁽⁺⁰⁾ | Paint: 1,701.15 ms -332.41 ms^(-16.3%)

Test	Duration	Renders
Slider mount (300 instances)	155.07 ms ▼-65.51 ms(-29.7%)	3 (+0)
Scroll Area mount (300 instances)	78.94 ms ▼-20.10 ms(-20.3%)	3 (+0)

10 tests within noise — details

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