# 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.