[poc] Label component

[atomiks]

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

Closes #4122

Preview: https://deploy-preview-4126--base-ui.netlify.app/react/components/label

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Not all SRs support an enclosing native <label> around a non-form (<span>) control. Therefore the demos either need to use aria-labelledby or a custom render function to render a label-wrapped <button> with nativeButton.

This component provides aria-labelledby automatically (same as Field.Label) to ensure the accessible name works broadly while supporting enclosing labels without invalid HTML. This also separates Field validation from labeling when controls need a basic label (especially Select/Combobox demos) where the native <label> has some undesirable defaults. We can neatly use this in all form control demos that need a lightweight label instead of using Field.

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This also adds a new useAriaLabelledby hook as a fallback for the sibling pattern without nativeButton, since it was common in Radix:

<label htmlFor="id" />
<Checkbox.Root id="id" />

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/@base-ui/react@4126
  

  pnpm add https://pkg.pr.new/@base-ui/utils@4126
  

commit: 85d0390

[mui-bot]

Bundle size report

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+1.32KB(+0.29%)	🔺+410B(+0.28%)

Details of bundle changes

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