feat(audio): Add Spectral Brush runtime (Phase 1 of Task #5)

Implement C++ runtime foundation for procedural audio tracing tool.

Changes:
- Created spectral_brush.h/cc with core API
  - Linear Bezier interpolation
  - Vertical profile evaluation (Gaussian, Decaying Sinusoid, Noise)
  - draw_bezier_curve() for spectrogram rendering
  - Home-brew deterministic RNG for noise profile
- Added comprehensive unit tests (test_spectral_brush.cc)
  - Tests Bezier interpolation, profiles, edge cases
  - Tests full spectrogram rendering pipeline
  - All 9 tests pass
- Integrated into CMake build system
- Fixed test_assets.cc include (asset_manager_utils.h)

Design:
- Spectral Brush = Central Curve (Bezier) + Vertical Profile
- Enables 50-100x compression (5KB .spec to 100 bytes C++ code)
- Future: Cubic Bezier, composite profiles, multi-dimensional curves

Documentation:
- Added doc/SPECTRAL_BRUSH_EDITOR.md (complete architecture)
- Updated TODO.md with Phase 1-4 implementation plan
- Updated PROJECT_CONTEXT.md to mark Task #5 in progress

Test results: 21/21 tests pass (100%)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

1 files changed, 497 insertions, 0 deletions

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+# Spectral Brush Editor (Task #5)
+
+## Concept
+
+The **Spectral Brush Editor** is a web-based tool for creating procedural audio by tracing spectrograms with parametric curves. It enables compact representation of audio samples for 64k demos.
+
+### Goal
+
+Replace large `.spec` asset files with tiny procedural C++ code:
+- **Before:** 5 KB binary `.spec` file
+- **After:** ~100 bytes of C++ code calling `draw_bezier_curve()`
+
+### Workflow
+
+```
+.wav file → Load in editor → Trace with spectral brushes → Export params + C++ code
+                                                               ↓
+                                                          (later)
+procedural_params.txt → Load in editor → Adjust curves → Re-export
+```
+
+---
+
+## Core Primitive: "Spectral Brush"
+
+A spectral brush consists of two components:
+
+### 1. Central Curve (Bezier)
+Traces a path through time-frequency space:
+```
+{freq_bin, amplitude} = bezier(frame_number)
+```
+
+**Properties:**
+- Control points: `[(frame, freq_hz, amplitude), ...]`
+- Interpolation: Linear (between control points)
+- Future: Cubic Bezier, Catmull-Rom splines
+
+**Example:**
+```javascript
+control_points: [
+  {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
+At each frame, applies a shape **vertically** in frequency bins around the central curve.
+
+**Profile Types:**
+
+#### Gaussian (Smooth harmonic)
+```
+amplitude(dist) = exp(-(dist² / σ²))
+```
+- **σ (sigma)**: Width in frequency bins
+- Use case: Musical tones, bass notes, melodic lines
+
+#### Decaying Sinusoid (Textured/resonant)
+```
+amplitude(dist) = exp(-decay * dist) * cos(ω * dist)
+```
+- **decay**: Falloff rate
+- **ω (omega)**: Oscillation frequency
+- Use case: Metallic sounds, bells, resonant tones
+
+#### Noise (Textured/gritty)
+```
+amplitude(dist) = random(seed, dist) * noise_amplitude
+```
+- **seed**: Deterministic RNG seed
+- Use case: Hi-hats, cymbals, textured sounds
+
+#### Composite (Combinable)
+```javascript
+{
+  type: "composite",
+  operation: "add" | "subtract" | "multiply",
+  profiles: [
+    {type: "gaussian", sigma: 30.0},
+    {type: "noise", amplitude: 0.1, seed: 42}
+  ]
+}
+```
+
+---
+
+## Visual Model
+
+```
+Frequency (bins)
+    ^
+    |
+512 |                    *         ← Gaussian profile (frame 80)
+    |                  * * *
+256 |          ****  *   *   *     ← Gaussian profile (frame 50)
+    |        **    **     * *
+128 |      **  Curve *      *      ← Central Bezier curve
+    |    **           *
+ 64 |  **              *
+    |  *                *
+  0 +--*--*--*--*--*--*--*--*---→ Time (frames)
+      0  10  20  30  40  50  60  70  80
+
+      At each frame:
+        1. Evaluate curve → get freq_bin_0 and amplitude
+        2. Draw profile vertically at that frame
+```
+
+---
+
+## File Formats
+
+### A. `procedural_params.txt` (Human-readable, re-editable)
+
+```text
+# Kick drum spectral brush definition
+METADATA dct_size=512 num_frames=100 sample_rate=32000
+
+CURVE bezier
+  CONTROL_POINT 0 200.0 0.9      # frame, freq_hz, amplitude
+  CONTROL_POINT 20 80.0 0.7
+  CONTROL_POINT 50 60.0 0.3
+  CONTROL_POINT 100 50.0 0.0
+  PROFILE gaussian sigma=30.0
+  PROFILE_ADD noise amplitude=0.1 seed=42
+END_CURVE
+
+CURVE bezier
+  CONTROL_POINT 0 500.0 0.5
+  CONTROL_POINT 30 300.0 0.2
+  PROFILE decaying_sinusoid decay=0.15 frequency=0.8
+END_CURVE
+```
+
+**Purpose:**
+- Load back into editor for re-editing
+- Human-readable, version-control friendly
+- Can be hand-tweaked in text editor
+
+### B. C++ Code (Ready to compile)
+
+```cpp
+// Generated from procedural_params.txt
+// File: src/audio/gen_kick_procedural.cc
+
+#include "audio/spectral_brush.h"
+
+void gen_kick_procedural(float* spec, int dct_size, int num_frames) {
+  // Curve 0: Low-frequency punch with noise texture
+  {
+    const float frames[] = {0.0f, 20.0f, 50.0f, 100.0f};
+    const float freqs[] = {200.0f, 80.0f, 60.0f, 50.0f};
+    const float amps[] = {0.9f, 0.7f, 0.3f, 0.0f};
+
+    draw_bezier_curve(spec, dct_size, num_frames,
+                      frames, freqs, amps, 4,
+                      PROFILE_GAUSSIAN, 30.0f);
+
+    draw_bezier_curve_add(spec, dct_size, num_frames,
+                          frames, freqs, amps, 4,
+                          PROFILE_NOISE, 0.1f, 42.0f);
+  }
+
+  // Curve 1: High-frequency attack
+  {
+    const float frames[] = {0.0f, 30.0f};
+    const float freqs[] = {500.0f, 300.0f};
+    const float amps[] = {0.5f, 0.2f};
+
+    draw_bezier_curve(spec, dct_size, num_frames,
+                      frames, freqs, amps, 2,
+                      PROFILE_DECAYING_SINUSOID, 0.15f, 0.8f);
+  }
+}
+
+// Usage in demo_assets.txt:
+// KICK_PROC, PROC(gen_kick_procedural), NONE, "Procedural kick drum"
+```
+
+**Purpose:**
+- Copy-paste into `src/audio/procedural_samples.cc`
+- Compile directly into demo
+- Zero runtime parsing overhead
+
+---
+
+## C++ Runtime API
+
+### New Files
+
+#### `src/audio/spectral_brush.h`
+```cpp
+#pragma once
+#include <cstdint>
+
+enum ProfileType {
+  PROFILE_GAUSSIAN = 0,
+  PROFILE_DECAYING_SINUSOID = 1,
+  PROFILE_NOISE = 2
+};
+
+// Evaluate linear Bezier interpolation at frame t
+float evaluate_bezier_linear(const float* control_frames,
+                              const float* control_values,
+                              int n_points,
+                              float frame);
+
+// Draw spectral brush: Bezier curve with vertical profile
+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);
+
+// Additive variant (for compositing profiles)
+void draw_bezier_curve_add(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);
+
+// Profile evaluation
+float evaluate_profile(ProfileType type, float distance,
+                      float param1, float param2);
+
+// Home-brew deterministic RNG (small, portable)
+uint32_t spectral_brush_rand(uint32_t seed);
+```
+
+**Future Extensions:**
+- Cubic Bezier interpolation: `evaluate_bezier_cubic()`
+- Generic loader: `gen_from_params(const PrimitiveData*)`
+
+---
+
+## Editor Architecture
+
+### Technology Stack
+- **HTML5 Canvas**: Spectrogram visualization
+- **Web Audio API**: Playback (IDCT → audio)
+- **Pure JavaScript**: No dependencies
+- **Reuse from existing editor**: `dct.js` (IDCT implementation)
+
+### Editor UI Layout
+
+```
+┌─────────────────────────────────────────────────────────────┐
+│  Spectral Brush Editor                          [Load .wav] │
+├─────────────────────────────────────────────────────────────┤
+│                                                               │
+│   ┌───────────────────────────────────────────────────┐     │
+│   │                                                     │     │
+│   │         CANVAS (Spectrogram Display)               │  T  │
+│   │                                                     │  O  │
+│   │  • Background: Reference .wav (gray, transparent)  │  O  │
+│   │  • Foreground: Procedural curves (colored)         │  L  │
+│   │  • Bezier control points (draggable circles)       │  S  │
+│   │                                                     │     │
+│   │                                                     │  [+]│
+│   │                                                     │  [-]│
+│   │                                                     │  [x]│
+│   └───────────────────────────────────────────────────┘     │
+│                                                               │
+├─────────────────────────────────────────────────────────────┤
+│  Profile: [Gaussian ▾]  Sigma: [████████░░] 30.0            │
+│  Curves: [Curve 0 ▾]    Amplitude: [██████░░░░] 0.8         │
+├─────────────────────────────────────────────────────────────┤
+│  [1] Play Procedural  [2] Play Original  [Space] Pause      │
+│  [Save Params] [Generate C++] [Undo] [Redo]                 │
+└─────────────────────────────────────────────────────────────┘
+```
+
+### Features (Phase 1: Minimal Working Version)
+
+#### Editing
+- Click to place Bezier control points
+- Drag to adjust control points (frame, frequency, amplitude)
+- Delete control points (right-click or Delete key)
+- Undo/Redo support (action history)
+
+#### Visualization
+- Dual-layer canvas:
+  - **Background**: Reference spectrogram (semi-transparent gray)
+  - **Foreground**: Procedural spectrogram (colored)
+- Log-scale frequency axis (musical perception)
+- Control points: Draggable circles with labels
+
+#### Audio Playback
+- **Key '1'**: Play procedural sound
+- **Key '2'**: Play original .wav
+- **Space**: Play/pause toggle
+
+#### File I/O
+- Load .wav or .spec (reference sound)
+- Save procedural_params.txt (re-editable)
+- Generate C++ code (copy-paste ready)
+
+#### Undo/Redo
+- Action history with snapshots
+- Commands: Add control point, Move control point, Delete control point, Change profile
+- **Ctrl+Z**: Undo
+- **Ctrl+Shift+Z**: Redo
+
+### Keyboard Shortcuts
+
+| Key | Action |
+|-----|--------|
+| **1** | Play procedural sound |
+| **2** | Play original .wav |
+| **Space** | Play/pause |
+| **Delete** | Delete selected control point |
+| **Esc** | Deselect all |
+| **Ctrl+Z** | Undo |
+| **Ctrl+Shift+Z** | Redo |
+| **Ctrl+S** | Save procedural_params.txt |
+| **Ctrl+Shift+S** | Generate C++ code |
+
+---
+
+## Implementation Plan
+
+### Phase 1: C++ Runtime (Foundation)
+**Files:** `src/audio/spectral_brush.h`, `src/audio/spectral_brush.cc`
+
+**Tasks:**
+- [ ] Define API (`ProfileType`, `draw_bezier_curve()`, etc.)
+- [ ] Implement linear Bezier interpolation
+- [ ] Implement Gaussian profile evaluation
+- [ ] Implement home-brew RNG (for future noise support)
+- [ ] Add unit tests (`src/tests/test_spectral_brush.cc`)
+
+**Deliverable:** Compiles, tests pass
+
+---
+
+### Phase 2: Editor Core
+**Files:** `tools/spectral_editor/index.html`, `script.js`, `style.css`, `dct.js` (reuse)
+
+**Tasks:**
+- [ ] HTML structure (canvas, controls, file input)
+- [ ] Canvas rendering (dual-layer: reference + procedural)
+- [ ] Bezier curve editor (place/drag/delete control points)
+- [ ] Profile controls (Gaussian sigma slider)
+- [ ] Real-time spectrogram rendering
+- [ ] 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, FFT/STFT → spectrogram)
+- [ ] Load .spec (binary format parser)
+- [ ] Save procedural_params.txt (text format writer)
+- [ ] Generate C++ code (code generation template)
+- [ ] Load procedural_params.txt (re-editing workflow)
+
+**Deliverable:** Full save/load cycle works
+
+---
+
+### Phase 4: Polish & Documentation
+**Tasks:**
+- [ ] UI refinements (tooltips, visual feedback)
+- [ ] Keyboard shortcut overlay (press '?' to show help)
+- [ ] Error handling (invalid files, audio context failures)
+- [ ] User guide (README.md in tools/spectral_editor/)
+- [ ] Example files (kick.wav → kick_procedural.cc)
+
+**Deliverable:** Production-ready tool
+
+---
+
+## Design Decisions
+
+### 1. Bezier Interpolation
+- **Current:** Linear (simple, fast, small code)
+- **Future:** Cubic Bezier, Catmull-Rom splines
+
+### 2. Parameter Limits
+- **Editor UI:** Soft ranges (reasonable defaults, not enforced)
+- **Examples:**
+  - Sigma: 1.0 - 100.0 (suggested range, user can type beyond)
+  - Amplitude: 0.0 - 1.0 (suggested, can exceed for overdrive effects)
+
+### 3. Random Number Generator
+- **Implementation:** Home-brew deterministic RNG
+- **Reason:** Independence, small code, repeatable results
+- **Algorithm:** Simple LCG or xorshift
+
+### 4. Code Generation
+- **Current:** Single function per sound (e.g., `gen_kick_procedural()`)
+- **Future:** Generic `gen_from_params(const PrimitiveData*)` with data tables
+
+### 5. Initial UI Scope
+- **Phase 1:** Bezier + Gaussian only
+- **Rationale:** Validate workflow before adding complexity
+- **Future:** Decaying sinusoid, noise, composite profiles
+
+---
+
+## Future Extensions
+
+### Bezier Enhancements
+- [ ] Cubic Bezier interpolation (smoother curves)
+- [ ] Catmull-Rom splines (automatic tangent control)
+- [ ] Bezier curve tangent handles (manual control)
+
+### Additional Profiles
+- [ ] Decaying sinusoid (metallic sounds)
+- [ ] Noise (textured sounds)
+- [ ] Composite profiles (add/subtract/multiply)
+- [ ] User-defined profiles (custom formulas)
+
+### Multi-Dimensional Bezier
+- [ ] `{freq, amplitude, oscillator_freq, decay} = bezier(frame)`
+- [ ] Per-parameter control curves
+
+### Advanced Features
+- [ ] Frequency snapping (snap to musical notes: C4, D4, E4, etc.)
+- [ ] Amplitude envelope presets (ADSR)
+- [ ] Profile library (save/load custom profiles)
+- [ ] Batch processing (convert entire folder of .wav files)
+
+### Code Generation
+- [ ] Generic `gen_from_params()` with data tables
+- [ ] Optimization: Merge similar curves
+- [ ] Size estimation (preview bytes saved vs. original .spec)
+
+---
+
+## Testing Strategy
+
+### C++ Runtime Tests
+**File:** `src/tests/test_spectral_brush.cc`
+
+**Test cases:**
+- Linear Bezier interpolation (verify values at control points)
+- Gaussian profile evaluation (verify falloff curve)
+- Full `draw_bezier_curve()` (verify spectrogram output)
+- Edge cases (0 control points, 1 control point, out-of-range frames)
+
+### Editor Tests
+**Manual testing workflow:**
+1. Load example .wav (kick drum)
+2. Place 3-4 control points to trace low-frequency punch
+3. Adjust Gaussian sigma
+4. Play procedural sound (should resemble original)
+5. Save procedural_params.txt
+6. Generate C++ code
+7. Copy C++ code into demo, compile, verify runtime output matches editor
+
+---
+
+## Size Impact Estimate
+
+**Example: Kick drum sample**
+
+**Before (Binary .spec):**
+- DCT size: 512
+- Num frames: 100
+- Size: 512 × 100 × 4 bytes = 200 KB (uncompressed)
+- Compressed (zlib): ~5-10 KB
+
+**After (Procedural C++):**
+```cpp
+// 4 control points × 3 arrays × 4 values = ~48 bytes of data
+const float frames[] = {0.0f, 20.0f, 50.0f, 100.0f};
+const float freqs[] = {200.0f, 80.0f, 60.0f, 50.0f};
+const float amps[] = {0.9f, 0.7f, 0.3f, 0.0f};
+draw_bezier_curve(...);  // ~20 bytes function call
+
+// Total: ~100 bytes
+```
+
+**Compression ratio:** 50-100× reduction!
+
+**Trade-off:** Runtime CPU cost (generation vs. lookup), but acceptable for 64k demo.
+
+---
+
+## References
+
+- **Bezier curves:** https://en.wikipedia.org/wiki/B%C3%A9zier_curve
+- **DCT/IDCT:** Existing implementation in `src/audio/dct.cc`
+- **Web Audio API:** https://developer.mozilla.org/en-US/docs/Web/API/Web_Audio_API
+- **Spectrogram visualization:** Existing editor in `tools/editor/` (to be replaced)