- 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.