Introduces a new "Object" entity type that blocks movement but not sight/bullets, distinct from walls and pickups:
- **Data**: New `objects` array in level JSON (`{x, y, frame}`), with `data/wolfenstein-objects.json` as the frame registry (Tall Bush, Gold Eagle)
- **Artwork**: New `objects.png` spritesheet registered in `wolfenstein-artwork.json`; placeholder texture via `paintObject()` in WolfensteinArt.js
- **Logic**: Objects stored in `state.objects`, with a derived `map.objectBlocked` Set checked only by movement collision (`isWallCell`) — raycaster/sight/bullets never read it, so objects are see-through and shoot-through by construction
- **View**: Rendered as billboard sprites in `_drawSprites` with ground-anchored positioning (bottom edge on floor) and asymptotic depth sorting that stays below the weapon viewmodel
- **Editor**: New "Object" tool category with dynamic dropdown; click open floor to place/remove (toggle), click wall is a no-op; teal square indicator on board
- **Validation**: `validateLevel` checks objects sit on open floor; save/load round-trips with graceful degradation for pre-existing saves
- **Tests**: Verifies object blocks player movement but bullets pass through
