chore(task): archive 08-03-use-low-poly-scrub-bush

.trellis/tasks/archive/2026-08/08-03-use-low-poly-scrub-bush/prd.md

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+# 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.