feat: migrate SpessaSynth → FluidSynth WASM (SF3 native, loop Gen 54 fix)

Replace SpessaSynth JS/AudioWorklet with FluidSynth C++ WASM
(@enikey87/fluidsynth-emscripten@0.1.1). CDN for dev, self-host
for prod. Fixes stuck Tremolo/Saxophone notes via compliant
Gen 44 loop mode processing. 100% audio parity with server
pyfluidsynth render (same C++ core).

- soundfontPlayer.js: FluidSynth WASM engine, preserve SonicSF API
- fluidsynthLoader.js: auto-select CDN (dev) vs self-host (prod)
- fluidsynth-bridge.js: PCM bridge AudioWorklet processor
- index.html: remove SpessaSynth importmap + module, add loader
- vendor/: libfluidsynth-2.3.0-sf3.js + .wasm (1.8MB self-host)

app/static/js/worklets/fluidsynth-bridge.js

@@ -0,0 +1,33 @@
+class FluidSynthBridge extends AudioWorkletProcessor {
+  constructor() {
+    super();
+    this.leftQ = [];
+    this.rightQ = [];
+    this.port.onmessage = (e) => {
+      const d = e.data;
+      if (d.type === 'PCM') {
+        this.leftQ.push(d.L);
+        this.rightQ.push(d.R);
+      }
+    };
+  }
+
+  process(inputs, outputs) {
+    const out = outputs[0];
+    if (!out) return true;
+    const len = out[0].length;
+    const l = this.leftQ;
+    const r = this.rightQ;
+    let ri = 0;
+    for (let i = 0; i < len; i++) {
+      if (ri >= l.length) { out[0][i] = 0; out[1][i] = 0; continue; }
+      out[0][i] = l[ri];
+      out[1][i] = r[ri];
+      ri++;
+    }
+    if (ri > 0) { this.leftQ.splice(0, ri); this.rightQ.splice(0, ri); }
+    return true;
+  }
+}
+
+registerProcessor('fluidsynth-bridge', FluidSynthBridge);