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>

.cargo/config.toml

@@ -1,4 +1,4 @@
-# quad-snd (macroquad audio) imports JS functions from mq_js_bundle.js, and our own
-# plugin (www/dc_plugin.js) provides more. Newer Rust versions need --allow-undefined for that.
+# Our JS plugin (www/dc_plugin.js) provides functions that the WASM module imports.
+# Newer Rust versions need --allow-undefined for imports that are not known at link time.
 [target.wasm32-unknown-unknown]
 rustflags = ["-C", "link-arg=--allow-undefined"]