fix: SF2 sound (audio sink FIFO underrun) + VST3 native GUI open + GUI load race

- bridgeAudioNode: ScriptProcessor 4096 drained only 1 of 16 blocks -> ~94% silence; now drains all queued blocks per callback, RING_DEPTH 8->24
- app.jsx: openNativeGUI had NO caller; wire after VST3 loadInstrument OK in track Synth dropdown + Plugin Manager Load Bridge
- main.cpp: OPEN_GUI polls up to 10s for LOAD completion (worker thread) before attach - fixes race 'no instrument loaded'
- index.html: bump bridgeAudioNode/app.precompiled cache versions

app/static/js/services/bridgeAudioNode.js

@@ -2,9 +2,12 @@
 // Audio sink: consumes 'bridge-audio' PCM frames (native bridge) and plays them
 // through a ScriptProcessorNode into the WebAudio graph (track FX / master bus).
 (function () {
-  var RING_DEPTH = 8;           // 8 blocks * 256 samples ~= 46ms anti-underrun
-  var SP_BUFFER = 4096;         // ScriptProcessor chunk (16 bridge blocks)
-  var _queue = [];
+  // Bridge renders one 256-sample block per ~5.8ms; ScriptProcessor fires once
+  // per 4096 samples (~16 blocks @44.1k). The ring MUST cover one full callback
+  // period or we underrun (silence gaps) — 24 blocks ~= 139ms headroom.
+  var RING_DEPTH = 24;
+  var SP_BUFFER = 4096;
+  var _chunks = [];       // [{l: Float32Array(256), r: Float32Array(256)}]
   var _spn = null;
   var _gainNode = null;
   var _ctx = null;
@@ -27,10 +30,26 @@
     _spn.onaudioprocess = function (e) {
       var L = e.outputBuffer.getChannelData(0);
       var R = e.outputBuffer.getChannelData(1);
+      var outLen = L.length;
       L.fill(0); R.fill(0);
-      if (!_queue.length) return;
-      var f = _queue.shift();
-      L.set(f.l); R.set(f.r);
+      if (!_chunks.length) return;
+      // Drain as many queued blocks as fit in this callback (16 max) — filling
+      // only one block per callback previously played 1/16 of the audio.
+      var written = 0;
+      while (written < outLen && _chunks.length) {
+        var blk = _chunks[0];
+        var n = Math.min(blk.l.length, outLen - written);
+        L.set(blk.l.subarray(0, n), written);
+        R.set(blk.r.subarray(0, n), written);
+        written += n;
+        if (n >= blk.l.length) {
+          _chunks.shift();
+        } else {
+          // Partial block (only possible if block size != 256): keep remainder.
+          blk.l = blk.l.subarray(n);
+          blk.r = blk.r.subarray(n);
+        }
+      }
     };
     _spn.connect(_gainNode);
     _initialized = true;
@@ -39,11 +58,12 @@
 
   function onAudio(l, r) {
     if (!_initialized) init();
-    _queue.push({ l: new Float32Array(l), r: new Float32Array(r) });
-    if (_queue.length > RING_DEPTH) _queue.shift(); // drop oldest on overrun
+    _chunks.push({ l: new Float32Array(l), r: new Float32Array(r) });
+    // Drop oldest on overrun — keeps latency bounded (~139ms max).
+    while (_chunks.length > RING_DEPTH) _chunks.shift();
   }
 
-  function flush() { _queue = []; }
+  function flush() { _chunks = []; }
 
   function getOutputNode() { return _gainNode; }