feat(figma): reorder sibling layers in the canonical DOM tree

[developer-1px]

Summary

• let non-root Figma Layers rows reorder within the same parent through native drag-and-drop
• add Alt+ArrowUp / Alt+ArrowDown using the shared Foundation target-placement grammar
• commit each reorder as one canonical document.apply move while preserving selection, focus, React DOM order, and history
• disable reorder for roots, cross-parent targets, boundaries/no-ops, and filtered search results
• reuse the existing layer rows and tokens with only transient insertion feedback
• expose keyboard shortcuts and repeatable live position announcements for assistive technology

Validation

• pnpm test — 416 files, 2,120 tests passed
• pnpm test:e2e — 124 tests passed
• pnpm lint
• pnpm exec tsc -b --pretty false
• pnpm build
• pnpm smoke:package
• pnpm pack:dry-run
• git diff --check
• three independent final reviews: no remaining blocker/P1/P2 findings

Closes #638