feat: add opt-in grid cell interaction

[gpbl]

NVDA in Chrome can announce a date twice when focus moves to a button inside a grid cell. This adds an opt-in dayInteraction="cell" mode that focuses the cell directly, while keeping dayInteraction="button" as the default.

Refs #3019.

What Changed

• Add cell focus, arrow navigation, and Enter/Space activation for single, multiple, range, and custom selection.
• Use shorter date names, expose weekday headers, associate grids with month captions, and convey selection and today through ARIA states. Preserve full date context for outside dates and hidden weekday headers.
• Add noninteractive DayContent customization and localizable labelKeyboardHelp instructions announced on grid entry.
• Add the CellInteraction comparison example, accessibility documentation, and regression coverage.

Review Notes

• Existing button markup, labels, and DayButton customization remain the default. The root role is unchanged.
• Cell mode reuses the day_button styling slot. Consumers opting in with custom button-tag selectors or custom components should follow the migration guidance in the accessibility guide.
• New component and label map entries are optional to preserve existing type assignments. Caption IDs include a fallback for React versions without useId.
• Keep this PR in draft pending manual NVDA/Chrome and VoiceOver testing of the final opt-in implementation. Open the examples app with ?example=CellInteraction to compare modes.

Validation

• pnpm lint
• pnpm typecheck
• pnpm test --runInBand: 223 suites passed; 1,160 tests passed, 1 todo; 3 snapshots passed.
• Final focused cell-interaction and ID-fallback tests: 70 passed.
• pnpm --filter examples-app build

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Latest commit: 663b9ae

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