feat: prove React FigJam widget seam

[developer-1px]

Summary

• publish the public React design authoring facade and immutable widget definition contract
• add the private first-party FigJam sticky-note and shape pack on the canonical DesignDocument/DOM editor path
• add isolated DOM-only fixture, inspector contribution, creation/transform/text/style/history coverage
• harden editor props previews and native editor-control selection boundaries

Verification

• npm test: 408 files, 2066 tests passed
• npm run test:e2e -- --project=chromium: 112 passed
• npm run lint
• npx tsc -b --pretty false
• npm run build
• npm run smoke:package
• npm run pack:dry-run -- --json
• git diff --check

Closes #627