Clean stale tuft density comment

.trellis/tasks/08-03-clean-tuft-density-comment/prd.md

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+# Clean stale tuft density comment
+
+## Goal
+
+Clean up the documented D3 comment debt around `tuft_density_wave()` so the
+code and Trellis specs no longer reference a deleted hedge-banding comparison.
+
+This is a documentation/comment correction only. It should not change Blender
+scene generation behavior or generated artifacts.
+
+## Background
+
+- `.trellis/spec/guides/artifact-parity-guide.md` records D3:
+  `generate_scene.py` `tuft_density_wave` has a stale comment comparing lawn
+  tuft density with deleted hedge banding.
+- `blender/generate_scene.py:354` defines `tuft_density_wave(x, y)`.
+- The current function body is still valid deterministic scatter logic; only
+  the explanatory comment is stale.
+
+## Requirements
+
+1. Update the comment above `tuft_density_wave()` to describe the current lawn
+   tuft spacing intent without referencing removed hedge banding behavior.
+2. Remove or update the D3 entry in
+   `.trellis/spec/guides/artifact-parity-guide.md` so it no longer lists this
+   as an outstanding known defect after the code comment is corrected.
+3. Do not change `tuft_density_wave()` math, tuft sampling thresholds, grass
+   material behavior, object names, counts, or scene output.
+4. Do not change D1 or D2.
+5. Do not change parity ignore lists.
+
+## Acceptance Criteria
+
+- [ ] `blender/generate_scene.py` no longer has a stale hedge-banding
+      comparison near `tuft_density_wave()`.
+- [ ] `.trellis/spec/guides/artifact-parity-guide.md` no longer records D3 as
+      outstanding.
+- [ ] `python3 -m py_compile blender/generate_scene.py` passes.
+- [ ] `python3 -m unittest blender/tests/test_pure.py` passes.
+- [ ] `git diff` shows no change to `tuft_density_wave()` executable code.
+
+## Out Of Scope
+
+- Any Blender/Cesium artifact-changing behavior.
+- Full parity capture, because the task is comment/spec-only and acceptance
+  explicitly verifies no executable code changed.
+- D1 / D2 cleanup.
+
+## Open Questions
+
+None.