Sync NAM engine block_size with JACK period on profile switch

- Add FastNAMHost.set_block_size() — reloads current model with
  new block_size, keeping the engine in sync with JACK's period
- Wire into POST /api/audio/profile so NAM engine restarts with
  correct frame count when switching between standard/low/stable
- Prevents sample-alignment drift when JACK period changes but
  NAM engine stays at old block_size (was causing passthrough)

src/dsp/nam_engine_host.py

@@ -93,6 +93,19 @@ class FastNAMHost:
             return 0.0
         return self._engine.avg_inference_ms
 
+    @property
+    def block_size(self) -> int:
+        return self._block_size
+
+    def set_block_size(self, block_size: int) -> None:
+        """Update block size. Reloads current model if loaded."""
+        if block_size == self._block_size:
+            return
+        self._block_size = block_size
+        if self._loaded_path:
+            logger.info("Block size changed to %d — reloading model %s", block_size, self._loaded_path)
+            self.load_model(self._loaded_path)
+
     # ── Model loading ──────────────────────────────────────────────
 
     def load_model(self, model_path: str) -> bool:

src/web/server.py

@@ -1361,6 +1361,10 @@ class WebServer:
             # Restart JACK audio client
             if jack_client:
                 jack_client.start()
+            # Sync NAM engine block size with new period
+            nam_host = self.deps.nam_host
+            if nam_host and hasattr(nam_host, 'set_block_size'):
+                nam_host.set_block_size(profile["period"])
             await self._manager.broadcast({
                 "type": "audio_profile_changed",
                 "profile": profile_key,