Add harness tooling and runtime polish

[agustin-littlehat]

Summary

• expand and reorganize the harness/browser fixture test surface
• ship runtime, menu, loading, and debug-flow polish already committed on the branch
• enable planar world components, improve atlas prep, and add source-face debug attribution
• include the gameplay follow-up fixes for model tightening holes and sigil target-use callbacks

Testing

• pnpm build