Adopt Shapespark vegetation assets

.trellis/tasks/08-03-test-shapespark-tree-variants/prd.md

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+# Test Shapespark tree variants
+
+## Goal
+
+Use the split Shapespark low-poly plant assets as tree, scrub bush, and lawn
+tuft variants, then validate `nantaizi-lake-innovation-valley`.
+
+The user liked the Shapespark `bush-03` scrub replacement and wants to evaluate
+whether the same kit can replace the current tree asset. If multiple tree
+variants are not too expensive, the scene should use varied trees while staying
+deterministic for the same OSM input.
+
+## Requirements
+
+1. Prefer Shapespark tree assets from
+   `assets/models/custom/shapespark_plants/tree-*/model.gltf`.
+2. Use deterministic variant selection based on stable tree instance ordering;
+   do not use unseeded random or time-dependent randomness.
+3. Preserve existing tree placement count and route/road/building behavior.
+4. Preserve the existing scrub bush `bush-03` experiment.
+5. Replace the old Poly Haven `shrub_02` lawn tuft asset with suitable
+   Shapespark grass assets.
+6. Evaluate the cost of loading multiple tree and lawn tuft variants:
+   - number of shared tree meshes introduced
+   - number of shared lawn tuft meshes introduced
+   - texture/image/material counts
+   - output GLB size
+   - total nodes
+7. Rebuild only `nantaizi-lake-innovation-valley` stages
+   `blender,cesium,preview`.
+
+## Acceptance Criteria
+
+- [ ] Tree generation uses Shapespark tree variants deterministically.
+- [ ] Lawn tufts use Shapespark `grass-*` variants instead of `shrub_02`.
+- [ ] `python3 -m py_compile blender/generate_scene.py blender/osmassets/tree.py` passes.
+- [ ] `python3 -m unittest blender/tests/test_pure.py` passes.
+- [ ] nantaizi `blender,cesium,preview` rebuild passes.
+- [ ] GLB digest reports tree mesh/material/image/node cost.
+- [ ] User can open the local preview and compare visual result.
+
+## Out Of Scope
+
+- Runtime LOD.
+- hanyang validation.
+- Texture compression / KTX2.
+- Committing the experiment before user approval.