docs: archive stale/completed docs, compact active refs (-1300 lines)

- Archive WORKSPACE_SYSTEM.md (completed); replace with 36-line operational ref
- Archive SHADER_REUSE_INVESTIGATION.md (implemented Feb 2026)
- Archive GPU_PROCEDURAL_PHASE4.md (completed feature)
- Archive GEOM_BUFFER.md (ideation only, never implemented)
- Archive SPECTRAL_BRUSH_EDITOR.md (v1 DCT approach, superseded by MQ v2)
- Update CLAUDE.md Tier 3 refs; point Audio to SPECTRAL_BRUSH_2.md
- Update TODO.md Task #5 design link to SPECTRAL_BRUSH_2.md
- Update COMPLETED.md archive index

handoff(Claude): doc cleanup done, 30 active docs (was 34), -1300 lines

1 files changed, 1 insertions, 1 deletions

diff --git a/TODO.md b/TODO.md
index b154926..aca1257 100644
--- a/TODO.md
+++ b/TODO.md
@@ -8,7 +8,7 @@
 
 Procedural spectrogram tool: 50-100× compression (5 KB .spec → ~100 bytes C++).
 
-**Design:** `doc/SPECTRAL_BRUSH_EDITOR.md`
+**Design:** `doc/SPECTRAL_BRUSH_2.md` (MQ-based v2)
 
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