Support template scans and room preset workflows

[wass08]

Summary

• add optional scan/guide preservation for template scene forks
• add room-preset save actions for selected zones, including zone-only vs zone-content deletion
• support room placement previews for ceilings and clear-underneath item previews
• keep zone thumbnails framed to the saved room footprint

Verification

• bunx biome check $(git diff --name-only origin/main..HEAD | rg '.(ts|tsx|md)$')
• bun test packages/core/src/utils/clone-scene-graph.test.ts packages/core/src/services/alignment.test.ts packages/core/src/services/alignment-anchors.test.ts
• bun run --cwd packages/core build
• bun run --cwd packages/editor check-types
• bun run --cwd packages/nodes build
• bun run --cwd packages/viewer build