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.toml

@@ -8,11 +8,6 @@ macroquad = { version = "0.4" }
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 
-[features]
-default = ["audio"]
-# macroquad audio (needs libasound natively); disable for native tests: cargo test --no-default-features
-audio = ["macroquad/audio"]
-
 [profile.release]
 opt-level = 3
 lto = true