V24: fix master-chain crackle, 1s lag, solo bypass

- worklet: FILL 48->24 (PDC 7936 dung), flush protocol khi resume sau interruption
  (clear queue + fade-in/crossfade chi voi block moi), drop block stale
- engine fx_realtime: _flush_input_locked NO-DROP (ring day -> giu pending, cap 2048),
  flush_all() dong bo in/out khi worklet resume
- app.jsx: WS flush_done + flushing guard, fallback timer 500ms
- plugins.py: xu ly {cmd:flush} -> flush_done
- index.html: bump v param

app/core/fx_realtime.py

@@ -130,7 +130,8 @@ class FxRealtimeSession:
         self.latencies = {}        # {slot_idx: samples}
         # Diagnostics (FXRT crackle hunt): drop/publish counters.
         self.stats = {'publish': 0, 'drop_batch': 0, 'drop_blocks': 0,
-                      'stale_flush': 0, 'read_blocks': 0, 'read_empty': 0}
+                      'stale_flush': 0, 'read_blocks': 0, 'read_empty': 0,
+                      'flush': 0}
         self._spawn()
 
     # ── process ─────────────────────────────────────────────────────────────
@@ -264,26 +265,34 @@ class FxRealtimeSession:
 
     def _flush_input_locked(self):
         """Publish pending vào input ring. Ghi data trước, in_write += n sau →
-        bridge đọc in_write 1 lần → thấy nguyên batch (take=4)."""
+        bridge đọc in_write 1 lần → thấy nguyên batch (take=4). V24 NO-DROP:
+        ring đầy → GIỮ pending (pump flush_input_stale 2ms retry) — không drop
+        → hết content gap + hết race pointer (drop cũ cập in_read vượt in_write
+        → avail > in_slots → ghi đè slot bridge đang đọc → torn frame). Cap
+        2048 block (~11s) chống tích lũy vô hạn khi client dồn liên tục."""
         if not self._in_pending:
             return
         h = self.h
+        if len(self._in_pending) > 2048:
+            drop = len(self._in_pending) - 2048
+            del self._in_pending[:drop]  # drop block CŨ NHẤT (front)
+            self.stats['drop_blocks'] += drop
         frames = self._in_pending
-        self._in_pending = []
-        self._in_pending_t0 = None
         avail = h.in_slots - (h.in_write - h.in_read)
         self.stats['publish'] += len(frames)
         if avail <= 0:
-            h.in_read += len(frames)  # ring full → drop cả batch
-            self.stats['drop_batch'] += len(frames)
+            # ring đầy → giữ nguyên pending; pump retry khi bridge drain
             return
         n = min(len(frames), avail)
-        drop = len(frames) - n
-        if drop > 0:
-            h.in_read += drop  # drop frame cũ nhất
-            self.stats['drop_blocks'] += drop
+        publish = frames[:n]
+        if n < len(frames):
+            del frames[:n]  # publish n block CŨ NHẤT, giữ phần thừa làm pending
+            self._in_pending_t0 = time.time() if frames else None
+        else:
+            self._in_pending = []
+            self._in_pending_t0 = None
         for i in range(n):
-            L, R = frames[i + drop]
+            L, R = publish[i]
             slot = (h.in_write + i) & (h.in_slots - 1)
             bl = IN_L_OFF // 4 + slot * self.block_size
             br = IN_R_OFF // 4 + slot * self.block_size
@@ -299,6 +308,17 @@ class FxRealtimeSession:
                 self.stats['stale_flush'] += 1
                 self._flush_input_locked()
 
+    def flush_all(self):
+        """V24: reset toàn bộ pipeline khi client resume sau stall — xóa
+        input/output ring + pending → bridge xử lí audio MỚI, không backlog cũ
+        (lag cộng dồn ~1s sau stall). Gọi khi nhận {cmd:'flush'} qua WS."""
+        with self._lock:
+            self._in_pending = []
+            self._in_pending_t0 = None
+            self.h.in_read = self.h.in_write
+            self.h.out_read = self.h.out_write
+            self.stats['flush'] += 1
+
     def read_output(self):
         """Drain output ring → list bytes (mỗi block 2048 bytes interleaved)."""
         out = []