feat: rename experimentalTransitionTypes to transitionTypes

[uhyo]

Closes #258

addTransitionType is stable as of React 19.3, so this drops the experimental prefix from the <Router> prop and updates the docs and code comments that still said React Canary was required.

Changes

• New transitionTypes prop on <Router>. experimentalTransitionTypes still works as a @deprecated alias, to be removed in the next major version. If both are passed, transitionTypes wins.
• Comments: Router/index.tsx, types.ts and core/addTransitionType.ts now say "React 19.3 or later" instead of "React Canary".
• Docs:
  • Transitions guide: uses the new prop name, and the "React Canary required" warning box is replaced with notes on the React 19.3+ requirement and the deprecated alias.
  • <Router> props table: adds a transitionTypes row. The prop wasn't listed there before.
• Tests: the existing tests now use transitionTypes. Two new tests cover the deprecated alias and which prop wins when both are passed.

Unchanged on purpose

• No-op fallback: the peer dependency still allows React 19.2, so on React versions without addTransitionType the prop is still called and the types are dropped.
• unstable_addTransitionType: the lookup still falls back to this older name, so Canary builds from before 19.3 keep working.

Verification

pnpm typecheck, pnpm lint, pnpm format:check and the router test suite (352 tests) all pass locally.

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https://claude.ai/code/session_01A82ucUxAtPLYyLo6hbguYA

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