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>

www/dc_plugin.js

@@ -1,4 +1,4 @@
-// DungeonCraft browser plugin for miniquad: storage, file download/upload, save-on-leave.
+// DungeonCraft browser plugin for miniquad: storage, file download/upload, save-on-leave, audio output.
 (function () {
     "use strict";
     var pending = null;      // value handed over by dc_storage_get
@@ -77,5 +77,44 @@
         };
     }
 
+    // ---- audio: the Rust mixer renders on demand (dc_audio_render), started by the first user input
+    var actx = null;
+    var stats = { frames: 0, peak: 0 };
+    // diagnostics, e.g. for automated tests: state of the audio output
+    window.dc_audio_stats = function () {
+        return { state: actx ? actx.state : "not started", frames: stats.frames, peak: stats.peak };
+    };
+    function startAudio() {
+        if (typeof wasm_exports === "undefined" || !wasm_exports.dc_audio_render) return;
+        if (actx) {
+            if (actx.state !== "running") actx.resume();
+            return;
+        }
+        var AC = window.AudioContext || window.webkitAudioContext;
+        if (!AC) return;
+        actx = new AC();
+        wasm_exports.dc_audio_init(actx.sampleRate);
+        var node = actx.createScriptProcessor(2048, 0, 2);
+        node.onaudioprocess = function (e) {
+            var n = e.outputBuffer.length;
+            var l = e.outputBuffer.getChannelData(0);
+            var r = e.outputBuffer.getChannelData(1);
+            var ptr = wasm_exports.dc_audio_render(n);
+            if (!ptr) { l.fill(0); r.fill(0); return; }
+            var buf = new Float32Array(wasm_memory.buffer, ptr, n * 2);
+            var peak = 0;
+            for (var i = 0; i < n; i++) {
+                l[i] = buf[2 * i]; r[i] = buf[2 * i + 1];
+                var a = Math.abs(l[i]); if (a > peak) peak = a;
+            }
+            stats.frames += n;
+            stats.peak = Math.max(stats.peak * 0.9, peak);
+        };
+        node.connect(actx.destination);
+    }
+    ["mousedown", "keydown", "touchstart"].forEach(function (ev) {
+        window.addEventListener(ev, startAudio, true);
+    });
+
     miniquad_add_plugin({ register_plugin: register, version: 1, name: "dungeoncraft" });
 })();