docs: Condense HOWTO.md, move details to technical docs

- HOWTO.md: 184→97 lines (quick reference only)
- BUILD.md: Add build modes, header organization, dependency tracking
- ASSET_SYSTEM.md: Add developer workflow section
- TRACKER.md: Add AudioEngine API documentation

Net: -147 lines in HOWTO.md

1 files changed, 27 insertions, 0 deletions

diff --git a/doc/BUILD.md b/doc/BUILD.md
index 1f65122..7cb3473 100644
--- a/doc/BUILD.md
+++ b/doc/BUILD.md
@@ -21,8 +21,17 @@ cmake --build build_final -j4
 | Mode | Flags | Use Case |
 |------|-------|----------|
 | Debug | `-DCMAKE_BUILD_TYPE=Debug` | Development, full error checking + debug features |
+| SIZE_OPT | `-DDEMO_SIZE_OPT=ON` | Size-optimized, with checks |
 | STRIP_ALL | `-DDEMO_STRIP_ALL=ON` | Release candidate, full error checking, no debug features (~64k target) |
 | FINAL_STRIP | `-DDEMO_FINAL_STRIP=ON` | Final release, no error checking, absolute minimum size |
+| STRIP_EXTERNAL_LIBS | `-DDEMO_STRIP_EXTERNAL_LIBS=ON` | Size measurement only (binary won't run) |
+
+**Build Hierarchy:**
+- **Debug:** Full checks + debug features (hot-reload, seek, etc.)
+- **STRIP_ALL:** Full checks, no debug (~64k target, always fullscreen)
+- **FINAL_STRIP:** No checks, no debug (absolute minimum, ⚠️ dangerous)
+
+**Note:** `DEMO_ALL_OPTIONS=ON` enables tests, tools, AND `STRIP_ALL`.
 
 ## Dependencies
 
@@ -67,3 +76,21 @@ open build/demo.xcodeproj
 ```
 
 Use Xcode Metal debugger for shader performance analysis.
+
+## Build System Internals
+
+**Asset Dependency Tracking:**
+- CMake tracks 42 demo + 17 test assets
+- Editing shaders/audio/sequences auto-triggers rebuild
+- Asset lists parsed to extract individual file dependencies
+
+**Header Organization:**
+- `asset_manager_dcl.h`: Forward declarations
+- `asset_manager.h`: Core API (GetAsset/DropAsset)
+- `asset_manager_utils.h`: Typed helpers
+
+**Code Generation:**
+- Timeline: `seq_compiler` → `generated/timeline.cc`
+- Music: `tracker_compiler` → `generated/music.cc`
+- Assets: `asset_packer` → `generated/assets.h` + `assets_data.cc`
+- Cross-compilation uses host-native tools for generation