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- Archive SPECTRAL_BRUSH_EDITOR.md (v1 DCT approach, superseded by MQ v2)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, 195 insertions, 0 deletions

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+# Spectral Brush Editor (Task #5)
+
+## Concept
+
+Replace large `.spec` assets with procedural C++ code (50-100× compression).
+
+**Before:** 5 KB binary `.spec` file
+**After:** ~100 bytes C++ code calling `draw_bezier_curve()`
+
+**Workflow:**
+```
+.wav → Load in editor → Trace with Bezier curves → Export procedural_params.txt + C++ code
+```
+
+---
+
+## Core Primitive: "Spectral Brush"
+
+### 1. Central Curve (Bezier)
+Traces time-frequency path: `{freq_bin, amplitude} = bezier(frame_number)`
+
+**Example control points:**
+```javascript
+[
+  {frame: 0, freq_hz: 200.0, amplitude: 0.9},   // Attack
+  {frame: 20, freq_hz: 80.0, amplitude: 0.7},   // Sustain
+  {frame: 100, freq_hz: 50.0, amplitude: 0.0}   // Decay
+]
+```
+
+### 2. Vertical Profile
+Shapes "brush stroke" around curve at each frame.
+
+**Profile Types:**
+- **Gaussian**: `exp(-(dist² / σ²))` - Musical tones, bass
+- **Decaying Sinusoid**: `exp(-decay * dist) * cos(ω * dist)` - Metallic sounds
+- **Noise**: `random(seed, dist) * amplitude` - Hi-hats, cymbals
+- **Composite**: Combine multiple profiles (add/subtract/multiply)
+
+---
+
+## File Formats
+
+### A. `procedural_params.txt` (Human-readable)
+```text
+METADATA dct_size=512 num_frames=100 sample_rate=32000
+
+CURVE bezier
+  CONTROL_POINT 0 200.0 0.9
+  CONTROL_POINT 20 80.0 0.7
+  CONTROL_POINT 100 50.0 0.0
+  PROFILE gaussian sigma=30.0
+END_CURVE
+```
+
+### B. C++ Code (Ready to compile)
+```cpp
+#include "audio/spectral_brush.h"
+
+void gen_kick_procedural(float* spec, int dct_size, int num_frames) {
+  const float frames[] = {0.0f, 20.0f, 100.0f};
+  const float freqs[] = {200.0f, 80.0f, 50.0f};
+  const float amps[] = {0.9f, 0.7f, 0.0f};
+
+  draw_bezier_curve(spec, dct_size, num_frames,
+                    frames, freqs, amps, 3,
+                    PROFILE_GAUSSIAN, 30.0f);
+}
+```
+
+---
+
+## C++ Runtime API
+
+### Files: `src/audio/spectral_brush.{h,cc}`
+
+**Key functions:**
+```cpp
+enum ProfileType {
+  PROFILE_GAUSSIAN,
+  PROFILE_DECAYING_SINUSOID,
+  PROFILE_NOISE
+};
+
+void draw_bezier_curve(float* spectrogram, int dct_size, int num_frames,
+                       const float* control_frames,
+                       const float* control_freqs_hz,
+                       const float* control_amps,
+                       int n_control_points,
+                       ProfileType profile_type,
+                       float profile_param1,
+                       float profile_param2 = 0.0f);
+
+float evaluate_bezier_linear(const float* control_frames,
+                              const float* control_values,
+                              int n_points,
+                              float frame);
+
+float evaluate_profile(ProfileType type, float distance,
+                      float param1, float param2);
+```
+
+---
+
+## Editor UI
+
+### Technology Stack
+- HTML5 Canvas (visualization)
+- Web Audio API (playback)
+- Pure JavaScript (no dependencies)
+- Reuse `dct.js` from existing editor
+
+### Key Features
+- Dual-layer canvas (reference + procedural spectrograms)
+- Drag control points to adjust curves
+- Real-time spectrogram rendering
+- Audio playback (keys 1/2 for procedural/original)
+- Undo/Redo (Ctrl+Z, Ctrl+Shift+Z)
+- Load .wav/.spec, save params, generate C++ code
+
+### Keyboard Shortcuts
+| Key | Action |
+|-----|--------|
+| 1 | Play procedural |
+| 2 | Play original |
+| Space | Play/pause |
+| Delete | Remove control point |
+| Ctrl+Z | Undo |
+| Ctrl+S | Save params |
+
+---
+
+## Implementation Phases
+
+### Phase 1: C++ Runtime
+**Files:** `src/audio/spectral_brush.{h,cc}`, `src/tests/test_spectral_brush.cc`
+
+**Tasks:**
+- Define API (ProfileType, draw_bezier_curve, evaluate_profile)
+- Implement linear Bezier interpolation
+- Implement Gaussian profile
+- Add unit tests
+- **Deliverable:** Compiles, tests pass
+
+### Phase 2: Editor Core
+**Files:** `tools/spectral_editor/{index.html, script.js, style.css, dct.js}`
+
+**Tasks:**
+- HTML structure (canvas, controls, file input)
+- Bezier curve editor (place/drag/delete control points)
+- Dual-layer canvas rendering
+- Real-time spectrogram generation
+- Audio playback (IDCT → Web Audio API)
+- Undo/Redo system
+- **Deliverable:** Interactive editor, can trace .wav files
+
+### Phase 3: File I/O
+**Tasks:**
+- Load .wav (decode, STFT → spectrogram)
+- Load .spec (binary parser)
+- Save procedural_params.txt (text format)
+- Generate C++ code (template)
+- Load procedural_params.txt (re-editing)
+- **Deliverable:** Full save/load cycle
+
+---
+
+## Design Decisions
+
+- **Bezier:** Linear interpolation (Phase 1), cubic later
+- **Profiles:** Gaussian only (Phase 1), others later
+- **Parameters:** Soft UI limits, no enforced bounds
+- **RNG:** Home-brew deterministic (small, repeatable)
+- **Code gen:** Single function per sound (generic loader later)
+
+---
+
+## Size Impact
+
+**Example: Kick drum**
+
+**Before (Binary):**
+- 512 bins × 100 frames × 4 bytes = 200 KB uncompressed
+- ~5 KB compressed (zlib)
+
+**After (Procedural):**
+- 4 control points × 3 arrays × 4 floats = ~48 bytes data
+- Function call overhead = ~20 bytes
+- **Total: ~100 bytes** (50-100× reduction)
+
+**Trade-off:** Runtime CPU cost, acceptable for 64k demo.
+
+---
+
+*See TODO.md for detailed implementation tasks.*