docs: Simplify all design docs (50% reduction, 1687 lines removed)

Consolidated and streamlined all documentation:

**Merged:**
- PROCEDURAL.md → deleted (content in ASSET_SYSTEM.md)
- FETCH_DEPS.md → BUILD.md (dependencies section)

**Simplified (line reductions):**
- HOWTO.md: 468→219 (53%)
- CONTRIBUTING.md: 453→173 (62%)
- SPECTRAL_BRUSH_EDITOR.md: 497→195 (61%)
- SEQUENCE.md: 355→197 (45%)
- CONTEXT_MAINTENANCE.md: 332→200 (40%)
- test_demo_README.md: 273→122 (55%)
- ASSET_SYSTEM.md: 271→108 (60%)
- MASKING_SYSTEM.md: 240→125 (48%)
- 3D.md: 196→118 (40%)
- TRACKER.md: 124→76 (39%)
- SCENE_FORMAT.md: 59→49 (17%)
- BUILD.md: 83→69 (17%)

**Total:** 3344→1657 lines (50.4% reduction)

**Changes:**
- Removed verbose examples, redundant explanations, unimplemented features
- Moved detailed task plans to TODO.md (single source of truth)
- Consolidated coding style rules
- Kept essential APIs, syntax references, technical details

**PROJECT_CONTEXT.md:**
- Added "Design Docs Quick Reference" with 2-3 line summaries
- Removed duplicate task entries
- All design docs now loaded on-demand via Read tool

Result: Context memory files reduced from 31.6k to ~15k tokens.

1 files changed, 62 insertions, 213 deletions

diff --git a/doc/test_demo_README.md b/doc/test_demo_README.md
index bbce023..6ccada0 100644
--- a/doc/test_demo_README.md
+++ b/doc/test_demo_README.md
@@ -1,273 +1,122 @@
-# test_demo - Audio/Visual Synchronization Debug Tool
+# test_demo - Audio/Visual Sync Debug Tool
 
-## Overview
-
-A minimal standalone tool for debugging audio/visual synchronization in the demo without the complexity of the full demo64k timeline.
+Minimal standalone tool for debugging audio/visual synchronization.
 
 ## Features
-
-- **Simple Drum Beat**: Kick-snare pattern with crash every 4th bar
-- **NOTE_A4 Reference Tone**: 440 Hz tone plays at start of each bar for testing
-- **Screen Flash Effect**: Visual feedback synchronized to audio peaks
-- **16 Second Duration**: 8 bars at 120 BPM
-- **Variable Tempo Mode**: Tests tempo scaling (1.0x ↔ 1.5x and 1.0x ↔ 0.66x)
-- **Peak Logging**: Export audio peaks to file for gnuplot visualization
+- Kick-snare drum pattern with crash every 4th bar
+- NOTE_A4 reference tone (440 Hz) at bar starts
+- Screen flash synchronized to audio peaks
+- 16 seconds (8 bars at 120 BPM)
+- Variable tempo mode (1.0x ↔ 1.5x / 0.66x)
+- Peak logging for gnuplot
 
 ## Building
-
 ```bash
 cmake --build build --target test_demo
 ```
 
 ## Usage
 
-### Basic Usage
-
+### Basic
 ```bash
 build/test_demo
 ```
 
 ### Command-Line Options
-
 ```
-  --help              Show help message and exit
-  --fullscreen        Run in fullscreen mode
-  --resolution WxH    Set window resolution (e.g., 1024x768)
-  --tempo             Enable tempo variation test mode
-  --log-peaks FILE    Log audio peaks at each beat (32 samples for 16s)
-  --log-peaks-fine    Log at each frame for fine analysis (~960 samples)
-                      (use with --log-peaks for millisecond resolution)
+--help              Show help
+--fullscreen        Fullscreen mode
+--resolution WxH    Window size (e.g., 1024x768)
+--tempo             Enable tempo variation
+--log-peaks FILE    Log audio peaks (beat-aligned)
+--log-peaks-fine    Log per-frame (~960 samples)
 ```
 
 ### Examples
-
-#### Run in fullscreen
 ```bash
+# Fullscreen
 build/test_demo --fullscreen
-```
 
-#### Custom resolution with tempo testing
-```bash
-build/test_demo --resolution 1024x768 --tempo
-```
+# Tempo test
+build/test_demo --tempo
 
-#### Log audio peaks for analysis (beat-aligned)
-```bash
+# Peak logging
 build/test_demo --log-peaks peaks.txt
-```
+gnuplot -p -e "plot 'peaks.txt' using 2:3 with lines"
 
-After running, visualize with gnuplot:
-```bash
-gnuplot -p -e "set xlabel 'Time (s)'; set ylabel 'Peak'; plot 'peaks.txt' using 2:3 with lines title 'Raw Peak'"
-```
-
-#### Log audio peaks with fine resolution (every frame)
-```bash
+# Fine-grained logging
 build/test_demo --log-peaks peaks_fine.txt --log-peaks-fine
 ```
 
-This logs at ~60 Hz (every frame) instead of every beat, providing millisecond-resolution data for detailed synchronization analysis. Produces ~960 samples for the 16-second demo.
-
-## Keyboard Controls
-
-- **ESC**: Exit the demo
-- **F**: Toggle fullscreen
+**Keyboard:** `Esc` (exit), `F` (toggle fullscreen)
 
 ## Audio Pattern
+8 bars, 120 BPM:
+- Bars 1-2, 4-6, 8: Kick-Snare-Kick-Snare + A4 tone
+- Bars 3, 7: Kick+Crash-Snare-Kick-Snare + A4 tone (landmarks)
 
-The demo plays a repeating drum pattern:
-
-```
-Bar 1-2: Kick-Snare-Kick-Snare (with A4 note)
-Bar 3:   Kick+Crash-Snare-Kick-Snare (with A4 note)  ← Crash landmark
-Bar 4-6: Kick-Snare-Kick-Snare (with A4 note)
-Bar 7:   Kick+Crash-Snare-Kick-Snare (with A4 note)  ← Crash landmark
-Bar 8:   Kick-Snare-Kick-Snare (with A4 note)
-```
-
-**Timing:**
-- 120 BPM (2 beats per second)
-- 1 bar = 4 beats = 2 seconds
-- Total duration: 8 bars = 16 seconds
-
-**Crash Landmarks:**
-- First crash at T=4.0s (bar 3, beat 8)
-- Second crash at T=12.0s (bar 7, beat 24)
-
-## Tempo Test Mode (`--tempo`)
+**Crash landmarks:** T=4.0s (bar 3), T=12.0s (bar 7)
 
-When enabled with `--tempo` flag, the demo alternates tempo scaling every bar:
+## Tempo Mode
+Alternates every bar:
+- Even bars: 1.0x → 1.5x (accelerate)
+- Odd bars: 1.0x → 0.66x (decelerate)
 
-- **Even bars (0, 2, 4, 6)**: Accelerate from 1.0x → 1.5x
-- **Odd bars (1, 3, 5, 7)**: Decelerate from 1.0x → 0.66x
+Music time drifts from physical time. Patterns respect tempo scaling.
 
-This tests the variable tempo system where music time advances independently of physical time:
-- `music_time += dt * tempo_scale`
-- Pattern triggering respects tempo scaling
-- Audio samples play at normal pitch (no pitch shifting)
+## Peak Logging
 
-**Console Output with --tempo:**
+**Beat-aligned** (32 samples):
 ```
-[T=0.50, MusicT=0.56, Beat=1, Frac=0.12, Peak=0.72, Tempo=1.25x]
-```
-
-**Expected Behavior:**
-- Physical time always advances at 1:1 (16 seconds = 16 seconds)
-- Music time advances faster during acceleration, slower during deceleration
-- By end of demo: Music time > Physical time (due to net acceleration)
-
-## Peak Logging Format
-
-The `--log-peaks` option writes a text file with three columns.
-
-### Beat-Aligned Mode (default)
-
-Logs once per beat (32 samples for 16 seconds):
-
-```
-# Audio peak log from test_demo
-# Mode: beat-aligned
-# To plot with gnuplot:
-#   gnuplot -p -e "set xlabel 'Time (s)'; set ylabel 'Peak'; plot 'peaks.txt' using 2:3 with lines title 'Raw Peak'"
 # Columns: beat_number clock_time raw_peak
-#
 0 0.000000 0.850000
 1 0.500000 0.720000
-2 1.000000 0.800000
-...
 ```
 
-**Columns:**
-1. **beat_number**: Beat index (0, 1, 2, ...)
-2. **clock_time**: Physical time in seconds
-3. **raw_peak**: Audio peak value (0.0-1.0+)
-
-### Fine-Grained Mode (`--log-peaks-fine`)
-
-Logs at every frame (approximately 960 samples for 16 seconds at 60 Hz):
-
+**Fine-grained** (~960 samples):
 ```
-# Audio peak log from test_demo
-# Mode: fine (per-frame)
-# To plot with gnuplot:
-#   gnuplot -p -e "set xlabel 'Time (s)'; set ylabel 'Peak'; plot 'peaks_fine.txt' using 2:3 with lines title 'Raw Peak'"
 # Columns: frame_number clock_time raw_peak beat_number
-#
 0 0.000000 0.850000 0
 1 0.016667 0.845231 0
-2 0.033333 0.823445 0
-3 0.050000 0.802891 0
-...
-30 0.500000 0.720000 1
-31 0.516667 0.715234 1
-...
 ```
 
-**Columns:**
-1. **frame_number**: Frame index (0, 1, 2, ...)
-2. **clock_time**: Physical time in seconds (millisecond precision)
-3. **raw_peak**: Audio peak value (0.0-1.0+)
-4. **beat_number**: Corresponding beat index (for correlation with beat-aligned mode)
-
-**Use Cases:**
-
-*Beat-Aligned Mode:*
-- Verify audio/visual synchronization at beat boundaries
-- Detect clipping at specific beats (peak > 1.0)
-- Analyze tempo scaling effects on pattern triggering
-- Compare expected vs actual beat times
-
-*Fine-Grained Mode:*
-- Millisecond-resolution synchronization analysis
-- Detect frame-level timing jitter or drift
-- Analyze audio envelope shape and attack/decay characteristics
-- Debug precise flash-to-audio alignment issues
-- Identify sub-beat audio artifacts or glitches
+**Use cases:**
+- Beat-aligned: Verify sync at beat boundaries, detect clipping
+- Fine-grained: Millisecond-resolution analysis, detect jitter
 
 ## Files
+- `src/test_demo.cc`: Main executable (~220 lines)
+- `assets/test_demo.track`: Drum pattern
+- `assets/test_demo.seq`: Visual timeline
+- `src/generated/test_demo_*.cc`: Generated code
 
-- **`src/test_demo.cc`**: Main executable (~220 lines)
-- **`assets/test_demo.track`**: Drum pattern and NOTE_A4 definition
-- **`assets/test_demo.seq`**: Visual timeline (FlashEffect)
-- **`src/generated/test_demo_timeline.cc`**: Generated timeline (auto)
-- **`src/generated/test_demo_music.cc`**: Generated music data (auto)
+## Verification
 
-## Verification Checklist
+**Normal mode:**
+- Flashes every ~0.5s, synchronized with kicks
+- Crash landmarks at T=4.0s and T=12.0s
+- No audio glitches
 
-### Normal Mode
-- [ ] Visual flashes occur every ~0.5 seconds
-- [ ] Audio plays (kick-snare drum pattern audible)
-- [ ] A4 tone (440 Hz) audible at start of each bar
-- [ ] Synchronization: Flash happens simultaneously with kick hit
-- [ ] Crash landmark: Larger flash + cymbal crash at T=4.0s and T=12.0s
-- [ ] Auto-exit: Demo stops cleanly at 16 seconds
-- [ ] Console timing: Peak values spike when kicks hit (Peak > 0.7)
-- [ ] No audio glitches: Smooth playback, no stuttering
+**Tempo mode:**
+- Bar 0 accelerates, bar 1 decelerates
+- Music time drifts from physical time
+- Smooth transitions
 
-### Tempo Test Mode
-- [ ] Bar 0 accelerates (1.0x → 1.5x)
-- [ ] Bar 1 decelerates (1.0x → 0.66x)
-- [ ] Music time drifts from physical time
-- [ ] Audio still syncs with flashes
-- [ ] Smooth transitions at bar boundaries
-- [ ] Physical time = 16s at end
-- [ ] Music time > 16s at end
-- [ ] Console shows tempo value
-
-### Peak Logging
-- [ ] File created when `--log-peaks` specified
-- [ ] Contains beat_number, clock_time, raw_peak columns
-- [ ] Gnuplot command in header comment
-- [ ] One row per beat (32 rows for 16 seconds)
-- [ ] Peak values match console output
-- [ ] Gnuplot visualization works
+**Peak logging:**
+- File created with correct format
+- Gnuplot visualization works
 
 ## Troubleshooting
 
-**Flash appears before audio:**
-- Cause: Audio latency too high
-- Fix: Reduce ring buffer size in `ring_buffer.h`
-
-**Flash appears after audio:**
-- Cause: Ring buffer under-filled
-- Fix: Increase pre-fill duration in `audio_render_ahead()`
-
-**No flash at all:**
-- Cause: Peak threshold not reached
-- Check: Console shows Peak > 0.7
-- Fix: Increase `visual_peak` multiplier in code (currently 8.0×)
-
-**A4 note not audible:**
-- Cause: NOTE_A4 volume too low or procedural generation issue
-- Check: Console shows correct sample count (4 samples)
-- Fix: Increase volume in test_demo.track (currently 0.5)
-
-**Peak log file empty:**
-- Cause: Demo exited before first beat
-- Check: File has header comments
-- Fix: Ensure demo runs for at least 0.5 seconds
+**Flash before audio:** Reduce ring buffer size (`ring_buffer.h`)
+**Flash after audio:** Increase pre-fill (`audio_render_ahead()`)
+**No flash:** Increase `visual_peak` multiplier (currently 8.0×)
+**A4 not audible:** Increase volume in `test_demo.track`
+**Empty log:** Ensure demo runs ≥0.5s
 
 ## Design Rationale
 
-**Why separate from demo64k?**
-- Isolated testing environment
-- No timeline complexity
-- Faster iteration cycles
-- Independent verification
-
-**Why use main demo assets?**
-- Avoid asset system conflicts (AssetId enum collision)
-- Reuse existing samples
-- No size overhead
-- Simpler CMake integration
-
-**Why 16 seconds?**
-- Long enough for verification (8 bars)
-- Short enough for quick tests
-- Crash landmarks at 25% and 75% for easy reference
-
-**Why NOTE_A4?**
-- Standard reference tone (440 Hz)
-- Easily identifiable pitch
-- Tests procedural note generation
-- Minimal code impact
+**Why separate?** Isolated testing, no timeline complexity
+**Why 16 seconds?** Long enough to verify, short enough to iterate
+**Why NOTE_A4?** Standard 440 Hz reference tone
+**Why crash landmarks?** Easy visual/audio synchronization verification