feat: avoid no-op draft writes

[jeremydw]

Summary

• Skip no-op draft updates before they enqueue Firestore writes
• Avoid metadata-only saves that can make unchanged published docs appear as drafts
• Add coverage for unchanged values, delete semantics, same-reference objects, and locale metadata arrays

Tests

• pnpm --filter @blinkk/root-cms exec vitest run ui/hooks/useDraftDoc.test.ts --exclude='**/*.visual.test.tsx'
• pnpm --filter @blinkk/root-cms build