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>

DESIGN.md

@@ -36,9 +36,12 @@ An original dungeon management game (genre: dungeon building & defence) that run
 - Settings: language + sound, camera (rotation/zoom speed, edge scrolling, invert mouse), graphics (tile outlines, wall detail, light sources, interface size), game (speed 1×/2×, hints), **key bindings** (conflicts swap keys; Esc and arrow keys are fixed)
 - Settings are stored in the browser **and** in every savegame; loading a savegame **always applies** its settings
 
-## Planned
-- Music: own **XM/MOD replayer in Rust**, shared mixer for music + sound effects (streamed via a JS audio plugin, replaces macroquad audio)
-- Soundtrack (composed as `.xm`, editable in MilkyTracker): title/menu, several calm build tracks, battle, victory/defeat jingles; **the title melody is a recognisable leitmotif in every piece**
+## Music & sound
+- Own **XM/MOD replayer in Rust** (`src/xm.rs`): instruments, envelopes, fadeout, auto-vibrato, linear/Amiga frequencies, common FT2 effects; verified against libopenmpt
+- **Shared mixer** for music + sound effects (`src/audio.rs`), streamed to the browser by the JS plugin (replaces macroquad audio)
+- Soundtrack in classic **Amiga chiptune** style, composed as `.xm` by `tools/compose_music.py` (editable in MilkyTracker): title/menu, **3 build tracks** (rotating), **2 battle tracks** (wave defence, gate assault), victory/defeat jingles; **the title melody is a recognisable leitmotif in every piece**
+- Music changes with **soft crossfades** (2.5 s): title → build → battle when heroes invade → storm music while attacking the gate → jingles
+- Sound effects are **synthesised chip sounds** (`src/sfx.rs`), positioned by distance and stereo panning
 
 ## Game content
 - **Digging**: mark earth/gold → Pit Imps dig, bring gold home and claim ground