Gap 8: PDC realtime — WS latency push + dry-path delay compensation

Backend: /ws/fx-realtime pump drain get_latencies (SHM ring, bridge da bao REPORT_LATENCY) -> push {cmd:'latency',total,latencies} khi tong doi. Frontend: pdcDelay (DelayNode) giua dryOutput->output bu delay master chain (round-trip 2 block + latency VST3), fxRtApplyPdc cap nhat tu WS frame, reset khi chain off. Test test_63 fake session 1024 -> frame total 1024; full 175 passed.

app/static/js/app.jsx

@@ -1061,7 +1061,13 @@ function initMasterBus(ctx) {
   const dryInput = ctx.createGain();
   const dryOutput = ctx.createGain();
   dryInput.connect(dryOutput);
-  dryOutput.connect(output);
+  // Gap 8 (PDC realtime): dry path bu delay master chain realtime — worklet
+  // round-trip ~2 block + latency VST3 (bridge bao qua WS {cmd:'latency'}).
+  // Bypassed tracks align voi mastered (fxRtApplyPdc cap nhat delayTime).
+  const pdcDelay = ctx.createDelay(5.0);
+  pdcDelay.delayTime.value = 0;
+  dryOutput.connect(pdcDelay);
+  pdcDelay.connect(output);
 
   const analyser = ctx.createAnalyser();
   analyser.fftSize = 256;
@@ -1101,6 +1107,7 @@ function initMasterBus(ctx) {
     masteringActive: false,
     dryInput,
     dryOutput,
+    pdcDelay,
     splitter,
     volSplit,
     volLGain,
@@ -1593,7 +1600,7 @@ function encodeFloatWavBlob(pcm, sampleRate, channels) {
 // worklet → analyser (sau master fader, trước metering/destination — khớp thứ
 // tự offline export). Track: mỗi track 1 session/worklet splice vào CẢ 2 đường
 // chain (audio-clips + MIDI/SF) — mọi luồng track qua bridge trước main out.
-const fxRt = { active: false, sessionId: null, ws: null, node: null, inQueue: [], pumpTimer: null, chainKey: null };
+const fxRt = { active: false, sessionId: null, ws: null, node: null, inQueue: [], pumpTimer: null, chainKey: null, latencyTotal: 0 };
 // Signature của chain VST realtime — phát hiện chain ĐỔI (insert/remove/bypass/
 // reorder VST) → restart session với chain mới (fxRtStart early-return khi
 // active, nếu không so chainKey thì VST mới insert không bao giờ được áp).
@@ -1684,6 +1691,18 @@ async function fxRtStart() {
     ws.binaryType = 'arraybuffer';
     ws.onopen = () => { fxRt.active = true; try { fxRtPushAllParams(); } catch (e) {} };
     ws.onmessage = (ev) => {
+      // Gap 8 (PDC realtime): text frame {cmd:'latency', total} tu server ->
+      // cap nhat delay dry-bypass path (bu latency chain VST3 realtime).
+      if (typeof ev.data === 'string') {
+        try {
+          const d = JSON.parse(ev.data);
+          if (d && d.cmd === 'latency' && typeof d.total === 'number') {
+            fxRt.latencyTotal = d.total;
+            fxRtApplyPdc();
+          }
+        } catch (e) {}
+        return;
+      }
       if (!(ev.data instanceof ArrayBuffer) || !fxRt.node) return;
       const f = new Float32Array(ev.data);
       const half = f.length / 2;
@@ -1749,11 +1768,28 @@ function fxRtStop(silent) {
   fxRtGuiPushShm();
   fxRt.active = false;
   fxRt.chainKey = null;
+  fxRt.latencyTotal = 0;
+  fxRtApplyPdc();
   if (sid) {
     try { window.SonicAPI.fxRealtimeStop({ session_id: sid }).catch(() => {}); } catch (e) {}
   }
 }
 
+// ── Gap 8: PDC realtime — bù delay dry-bypass path (master chain) ──────────
+// Master chain (worklet → bridge VST3) trễ ~2 block round-trip + latency
+// plugin. Track bypass mastering đi dry path (dryInput→dryOutput→output) KHÔNG
+// qua worklet → lệch pha với mastered. fxRtApplyPdc trễ dry path đúng lượng
+// để align. FX_RT_ROUNDTRIP_SAMPLES là ước lượng (~2 block 256 @44.1k) —
+// ponytail: đo RTT thật (tag seq trên block) khi cần sample-accurate.
+const FX_RT_ROUNDTRIP_SAMPLES = 512;
+function fxRtApplyPdc() {
+  const ctx = getAudioContext();
+  const bus = masterBus;
+  if (!ctx || !bus || !bus.pdcDelay) return;
+  const l = (fxRt && fxRt.active) ? (FX_RT_ROUNDTRIP_SAMPLES + (fxRt.latencyTotal || 0)) : 0;
+  try { bus.pdcDelay.delayTime.setTargetAtTime(l / (ctx.sampleRate || 44100), ctx.currentTime, 0.05); } catch (e) {}
+}
+
 // ── Realtime VST FX per-track (SHM bridge) — mọi luồng track qua bridge ─────
 // Mỗi track: session riêng (1 WS/SHM) cho TỪNG đường chain có âm — audio-clips
 // (path 'audio', splice fxEntry→mods→worklet→fxLegacyIn) và MIDI/SF (path 'sf',