feat(figjam): cut /figjam over to the canonical React DOM editor

[developer-1px]

Summary

• move /figjam onto the shared DesignDocument, ReactDesignRenderer, DomProjection, EditorEngine, viewport, preview, and history runtime used by /figma
• replace the legacy CanvasItem route with a private React FigJam product and 12 widget plus 3 structural definitions covering all 21 checked product exposures and aliases
• keep drawings and connectors as direct DOM roots with widget-owned SVG geometry, stable node references, reference-safe clipboard/delete/reparent behavior, and localStorage restoration
• add content-first creation, selection, marquee, move/resize capabilities, direct text editing, drawing/eraser, connector attachment, grouping/sectioning, imports, keyboard commands, pan/zoom, and hidden palette families
• remove the /figjam-widgets fixture route while retaining public pre-1.0 CanvasItem compatibility exports and the separate /engine compatibility route
• add the truthful migration matrix and architecture guards so future families do not require engine unions, renderer switches, or product conditionals

Verification

• pnpm test: 416 files, 2116 tests passed
• pnpm test:e2e: 123 passed
• pnpm lint
• pnpm exec tsc -b --pretty false
• pnpm build
• pnpm smoke:package
• pnpm pack:dry-run
• git diff --check

No version bump, tag, package publication, multiplayer backend, or marketplace work is included.

Closes #628