Phase 4+5: WebAudio I/O shim + verify DAW checklist — live=export, order, fader, PDC, smoke

app/core/fx_realtime.py

@@ -140,6 +140,8 @@ class FxRealtimeSession:
                 "type": s.get("type", ""),
                 "bypass": bool(s.get("bypass")),
             }
+            if s.get("id"):
+                slot["id"] = s["id"]
             if s.get("path"):
                 slot["path"] = s["path"]
             if s.get("preset_b64"):

app/core/render_engine.py

@@ -304,7 +304,11 @@ def _merge_track_fx_chain(track):
         if isinstance(slot, dict):
             merged.append(slot)
     if not track.get("vst_fx_bypass"):
-        merged += list(track.get("vst_fx_chain") or [])
+        seen = {s.get("path") for s in merged if s.get("path")}
+        for slot in (track.get("vst_fx_chain") or []):
+            if isinstance(slot, dict) and slot.get("path") in seen:
+                continue  # đã có trong fx_chain (migration) — không nhân đôi
+            merged.append(slot)
     return merged
 
 # Param mapping master chain entry → C++ BuiltinFx (Phase 3.2). UI chain entry