feat(world): authored grid-snap placement pass

[f1cklepickle]

Why

• Establish a reliable authored placement workflow on the 1x1 world grid baseline so scene composition stays deterministic as P3 systems expand.

Proposed Change

• Validate and document editor grid-snap workflow for world object placement in MainPrototype.
• Add/adjust placement guards so authored objects align cleanly to the 1x1 grid.
• Keep scope to placement workflow and validation (no new tileset art creation in this issue).

Acceptance Criteria

• World objects can be placed using grid snap and land on 1x1 cell boundaries.
• Authored placement remains visually aligned after Play/Stop and scene reload.

Testing / Validation

• Manual:
  • Enable editor grid snapping, move representative world objects, and verify alignment to 1x1 grid in MainPrototype.
• Automated:
  • EditMode: N/A
  • PlayMode: N/A

Context (Optional)

• Related: Establish 32×32 World Grid Baseline #128