[select][combobox][slider] Add Label parts

[atomiks]

• I have followed (at least) the PR section of the contributing guide.

The Label part is added for components that render a <div> label instead of using <label>. This matches Progress and Meter, and removes the need to use Field.

Forms: https://deploy-preview-4167--base-ui.netlify.app/react/handbook/forms#naming-form-controls

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@4167
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@4167
  

commit: c9bb899

[mui-bot]

Bundle size report

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+1.7KB(+0.37%)	🔺+510B(+0.34%)

Details of bundle changes

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[atomiks]

Codex Review

Overview

This patch adds dedicated Label parts for Select, Combobox, and Slider, and updates the shared label plumbing so button-based controls can be labeled without routing everything through Field.Label. It also updates the public docs and handbook examples so the guidance matches the final standalone labeling APIs, including the grouped-slider guidance.

Findings

No blocking issues found in this patch.

1. ℹ️ [Informational] Focused SSR render benchmark remains effectively flat

Impact: The extra label registration and aria-labelledby resolution work in the shared paths does not look like a meaningful render-time regression, so performance is not a blocker here.

Evidence: The hot paths touched by this branch are in packages/react/src/labelable-provider/useLabel.ts, packages/react/src/select/trigger/SelectTrigger.tsx, packages/react/src/combobox/trigger/ComboboxTrigger.tsx, and packages/react/src/slider/root/SliderRoot.tsx. I reran a focused renderToString benchmark on the latest master versus the current PR head, rendering 50 each of Field.Label + Select, Field.Label + Combobox, and Field.Label + Slider per sample.

Ref	Mean (ms)	p50 (ms)	p95 (ms)	Sample size
master (e90b027)	13.53	13.12	16.82	30
PR head (e87fdc4)	13.25	12.88	16.94	30

Recommendation: No perf-specific change is needed.

Confidence: 4/5

High confidence based on a full master...HEAD pass over both runtime code and docs, fresh targeted JSDOM coverage for FieldLabel, ComboboxLabel, ComboboxRoot, SelectLabel, SelectRoot, SliderLabel, and SliderRoot, pnpm docs:validate, and the focused SSR render benchmark above. Residual risk is mainly in unusual label compositions that are not yet covered by explicit tests.

Notes

• Docs were included in this pass: the Select, Combobox, and Slider component pages, the forms handbook page, the generated handbook index, and the updated demos/examples were all rechecked against the shipped API.
• Latest delta since the previous review: the forms handbook heading structure was flattened, the implicit-label guidance now sits with the naming guidance instead of the description section, and the generated handbook outline was updated to match.