feat(react-design): add reusable component engine and mobile samples

[developer-1px]

Summary

• Add three realistic 390×844 mobile travel frames while preserving the existing minimal visual language.
• Add a headless, subscribable component definition catalog with last-valid fallback for invalid live updates.
• Let Story-like host adapters register React component definitions without coupling the engine to CSF, Vite, routes, or providers.
• Expose linked component instances from DesignDocument and support explicit Instance or Definition edit propagation with atomic undo.
• Route Inspector fields and direct-manipulation previews through the selected component edit scope.
• Add iframe DOM measurement projection so host overlays can share client coordinates with inner DOM.
• Demonstrate the engine in the existing Figma clone using current designs and controls rather than a separate product shell.

Verification

• pnpm lint
• pnpm build
• pnpm smoke:package
• pnpm smoke:package:tarball
• Full Vitest suite: 436 files, 2,253 tests passed
• Public consumer, live registry, iframe projection, component scope, and React event integration tests passed
• In-app browser visual QA was unavailable in the current session; CI Playwright remains the browser gate

Commits

• d739046 feat(figma): add realistic mobile travel samples
• 8bc47b8 feat(react-design): add editable component slots
• 0cfa188 feat(react-design): add component definition sources