- Implement `StageTargetCaptureSession` for managing target capture sessions with throw and set down positions.
- Create `StageTerrainPainter` for terrain drag painting and overlap classification with stroke preview.
- Introduce `StageThemeController` to handle theme values and resource loading for the Sandbox Stage.
- Develop `StageRuleEngine` for runtime event matching and collision bookkeeping.
- Add `StageSpatialIndex` for broadphase stage lookups with advisory spatial indexing.