# Use low-poly scrub bush asset

## Goal

Prefer the new custom low-poly scrub bush asset for scrub edge instances and
validate `nantaizi-lake-innovation-valley` Blender/Cesium/preview output.

The user supplied `assets/models/custom/bush_low/bush_low.glb`. The goal is to
test whether using it reduces scrub bush mesh cost and improves Follow
smoothness without losing scrub ground cover or breaking material tinting.

## Requirements

1. Update `blender/generate_scene.py` to prefer
   `assets/models/custom/bush_low/bush_low.glb` for scrub edge bushes.
2. Keep `assets/models/custom/bush/bush.glb` as a fallback if the low asset is
   missing or fails to import.
3. Preserve deterministic scrub edge sampling and current scrub density budget:
   `SCRUB_BUSH_SPACING = 1.8`, `SCRUB_BUSH_LIMIT_PER_PATCH = 60`.
4. Preserve existing material tint behavior based on material names containing
   `leaf`, `trunk`, or `twig`.
5. Rebuild `nantaizi-lake-innovation-valley` stages `blender,cesium,preview`.
6. Measure and report:
   - `SCENE_DONE.scrub_bushes`
   - GLB total nodes
   - scrub bush node count
   - scrub bush mesh vertex/index bytes or primitive counts from GLB digest
   - output GLB size
7. Do not change road/building/tree/vehicle route behavior.

## Acceptance Criteria

- [ ] `generate_scene.py` prefers `bush_low.glb` and falls back to `bush.glb`.
- [ ] `python3 -m py_compile blender/generate_scene.py` passes.
- [ ] `python3 -m unittest blender/tests/test_pure.py` passes.
- [ ] `npm run build:area -- --config config/areas/nantaizi-lake-innovation-valley.json --stages blender,cesium,preview` passes.
- [ ] GLB digest confirms scrub bush nodes remain about `266` and scrub ground
      metadata remains present.
- [ ] Task report/changelog records whether `bush_low` meaningfully reduced
      mesh cost and whether it is worth keeping.

## Out Of Scope

- Compressing textures or changing baked material export.
- Replacing the preview car asset.
- Runtime LOD implementation.
- Further changing scrub density unless low asset validation shows it is needed.