fix(fx-rt): burst 4-block input batch qua bridge — hết lag/giật/crackle master FX

- worklet gom 1024 mẫu (4 block) vào 1 frame interleaved, main thread tách 4 WS frame 512 floats
- bridge take=min(avail,4) xử lí 1 call process cho burst → overhead VST3 amortize 4 lần
- engine _pump_output pace theo deadline thay vì sleep cố định (outRead 131→187.5/s)
- FX_RT_ROUNDTRIP_SAMPLES 512→1792 (RTT batching ~7 block)

app/api/v1/plugins.py

@@ -1071,6 +1071,7 @@ async def ws_fx_realtime(websocket: WebSocket, session_id: str):
         # de bu delay dry-bypass path (app.jsx fxRtApplyPdc).
         _last_lat = None
         try:
+            _loop = asyncio.get_event_loop()
             while True:
                 try:
                     _lats = sess.get_latencies()
@@ -1088,9 +1089,17 @@ async def ws_fx_realtime(websocket: WebSocket, session_id: str):
                 if not blks:
                     await asyncio.sleep(0.002)
                     continue
-                for blk in blks:
+                # Pace theo deadline (không sleep cố định sau mỗi send): thời
+                # gian send_bytes (WS → browser) CỘNG sleep cố định làm chu kì
+                # mỗi block > block_sec → engine drain chậm hơn realtime
+                # (đo: outRead 131/s thay vì 187.5/s → worklet zero-fill crackle).
+                # Gửi mỗi block cách nhau đúng block_sec kể từ mốc t0.
+                _t0 = _loop.time()
+                for _i, blk in enumerate(blks):
                     await websocket.send_bytes(blk)
-                    await asyncio.sleep(block_sec)
+                    _dt = _t0 + (_i + 1) * block_sec - _loop.time()
+                    if _dt > 0:
+                        await asyncio.sleep(_dt)
         except Exception:
             pass
     pump_task = asyncio.create_task(_pump_output())