fix: support React 19 in v8

[gpbl]

This v8 maintenance patch lets existing v8 consumers install DayPicker with React 19 while keeping the runtime API unchanged.

What Changed

• Bumped react-day-picker to 8.10.2.
• Expanded the React peer dependency to include ^19.0.0 while keeping the existing React 16.8, 17, and 18 support range.
• Replaced exported JSX.* declaration references with React types that remain valid across React 16.8 through React 19.
• Kept type-only React imports as import type in shipped src/ files, so downstream source transpilation cannot preserve type-only React imports as runtime imports.
• Added v8 README and changelog notes for React 19 compatibility.
• Added a v8 Trusted Publishing release workflow that publishes with npm publish --tag legacy-v8 and relies on npm Trusted Publishing for provenance.
• Added release-time packed smoke tests for React 19 runtime installs, React 19 TypeScript declarations, and React 18 TypeScript declarations with date-fns 2 and 3.
• Moved the release smoke fixtures into .github/scripts/ so the workflow remains readable while keeping the same validation coverage.
• Added a release tag/package version guard before publish.
• Removed the old token-based publish job from the package workflow.
• Updated the one source test import that used react-dom/test-utils for act.

Review Notes

Validation run locally on the v8 maintenance branch:

• corepack pnpm@8.6.2 typecheck
• corepack pnpm@8.6.2 lint --max-warnings=0
• corepack pnpm@8.6.2 test
• corepack pnpm@8.6.2 build
• Confirmed built declarations contain no JSX.* references
• Packed tarball comparison against v8.10.1: shipped JS and CSS runtime files have identical hashes; differences are limited to declarations, source maps/source text, docs, and package metadata
• Packed consumer runtime smoke test with react@19, react-dom@19, and date-fns@3
• Packed consumer runtime smoke test with react@19, react-dom@19, and date-fns@2
• Packed consumer TypeScript smoke test with react@19, react-dom@19, @types/react@19, @types/react-dom@19, typescript@5, date-fns@3, and skipLibCheck: false
• Packed consumer TypeScript smoke test with react@16.14, react-dom@16.14, @types/react@16.14, @types/react-dom@16.9, typescript@4.9, date-fns@2, and skipLibCheck: false
• Packed React 18 parity smoke against both 8.10.1 and 8.10.2, with date-fns@2 and date-fns@3; each case typechecked with skipLibCheck: false and rendered <DayPicker /> in jsdom
• npm publish --dry-run --tag legacy-v8

After merge, publish by creating the v8.10.2 GitHub Release from the merged maintenance commit. The npm publish should run from the release event, not from a manual workflow dispatch.

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