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, 13 insertions, 61 deletions

diff --git a/doc/TRACKER.md b/doc/TRACKER.md
index f3a34a3..6e71951 100644
--- a/doc/TRACKER.md
+++ b/doc/TRACKER.md
@@ -1,56 +1,17 @@
 # Minimal Audio Tracker
 
-in addition to being able to generate spectrograms (aka "Samples") on the
-fly and play them at once, we need a way to assemble samples (assets or 
-generated) into modifiable patterns and loops. Like what the Trackers were
-doing in the old time.
-
-## The idea
-
-A script can take a 'tracker music' text file that describes the sequence
-of samples (a 'pattern') in a first part. In a second part, these sequence
-a laid out with a timestamp (potentially overlapping) to generate the full
-music score.
-
-The patterns' samples (spectrograms) are not yet generated, it's just the
-'musical score' here. We still need to 'play' the score but with modifiers
-applied.
-
-### Modifiers
-
-For diversity, these essential patterns can be modified on the fly before
-being generated as spectrogram on the fly.
-Modifiers could be:
-  * randomize (for drums, e.g.)
-  * add accents and stresses
-  * modulate volume
-  * add distortion or noise
-  * add 'grain' and static noise
-  * flanger, vocoding, robotic voice, etc.
-  * etc.
-
-These modifiers are applied to a predefined pattern just before it's
-generated for playback (or assembling in the final track).
-
-### How would that work in practice?
-
-The musical score is a text file, that a tool will convert to run-time
-code to be compiled in the final binary demo64k.
-This generated code can be mixed with fixed code from the demo codebase
-itself (explosion predefined at a given time ,etc.)
-The baking is done at compile time, and the code will go in src/generated/
+System for assembling audio samples (assets or procedural) into patterns and loops.
 
 ## .track File Format
 
-### Timing System
+### Timing Convention
 
-**Unit-less Timing Convention:**
-- All time values are **unit-less** (not beats or seconds)
-- Convention: **1 unit = 4 beats**
-- Conversion to seconds: `seconds = units * (4 / BPM) * 60`
+**Unit-less timing:**
+- **1 unit = 4 beats**
+- Conversion: `seconds = units * (4 / BPM) * 60`
 - At 120 BPM: 1 unit = 2 seconds
 
-This makes patterns independent of BPM - changing BPM only affects playback speed, not pattern structure.
+Patterns are BPM-independent. Changing BPM only affects playback speed.
 
 ### File Structure
 
@@ -96,29 +57,20 @@ PATTERN melody LENGTH 1.0
   0.50, NOTE_G4, 0.8, 0.0
 ```
 
-### Conversion Reference
-
-At 120 BPM (1 unit = 4 beats = 2 seconds):
-
-| Units | Beats | Seconds | Description |
-|-------|-------|---------|-------------|
-| 0.00  | 0     | 0.0     | Start       |
-| 0.25  | 1     | 0.5     | Quarter     |
-| 0.50  | 2     | 1.0     | Half        |
-| 0.75  | 3     | 1.5     | Three-quarter |
-| 1.00  | 4     | 2.0     | Full pattern |
-
 ### Pattern Length
 
-- `LENGTH` parameter is optional, defaults to 1.0
-- Can be any value (0.5 for half-length, 2.0 for double-length, etc.)
-- Events must be within range `[0.0, LENGTH]`
+- `LENGTH` parameter defaults to 1.0 if omitted
+- Can be any value (0.5 for half-length, 2.0 for double-length)
+- Events must be within `[0.0, LENGTH]`
 
-Example of half-length pattern:
+Example half-length pattern:
 ```
 PATTERN short_fill LENGTH 0.5   # 2 beats = 1 second at 120 BPM
   0.00, ASSET_HIHAT, 0.7, 0.0
   0.50, ASSET_HIHAT, 0.6, 0.0   # 0.50 * 0.5 = 1 beat into the pattern
 ```
 
+### Future: Modifiers
 
+Potential runtime modifiers (not yet implemented):
+- Randomization, accents, volume modulation, distortion, noise, effects