Own XM/MOD replayer, audio mixer with crossfades, synthesised chip sound effects, music director

- src/xm.rs: FastTracker 2 XM and ProTracker MOD replayer (verified against libopenmpt)
- src/audio.rs: music players with crossfade + effect voices, rendered on demand
- src/sfx.rs: 29 synthesised chip sound effects
- www/dc_plugin.js: AudioContext started on first input, streams samples from WASM
- music follows the game: title, rotating build tracks, battle, gate assault, jingles
- macroquad audio removed; all tests run natively with cargo test

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -34,12 +34,16 @@ python3 -m http.server 8080 -d www   # open http://localhost:8080
 
 ### Testing the game logic
 
-The game logic (`src/game.rs`, `src/map.rs`) does not depend on macroquad. The tests (a headless simulation of whole games and a savegame round trip) run natively without the audio feature:
+The tests run natively: a headless simulation of whole games, a savegame round trip, rendering of all music modules and the audio engine:
 
 ```sh
-cargo test --release --no-default-features
+cargo test --release
 ```
 
+## Music
+
+The soundtrack consists of FastTracker 2 modules in `assets/music/`, generated by `tools/compose_music.py` (`python3 tools/compose_music.py`). They can be opened and edited in MilkyTracker. The game plays them with its own XM/MOD replayer (`src/xm.rs`); sound effects are synthesised chip sounds (`src/sfx.rs`). Both are mixed in `src/audio.rs` and streamed to the browser by `www/dc_plugin.js`.
+
 ## Controls
 
 | Input | Action |