fix(export): carry bridge-captured VSTi preset into offline render

Offline render tao bridge moi khong co preset -> plugin load init patch
-> am khac realtime (user tuong fallback). Gio doc presetBase64 tu
bridge_state.json (channel == track.id, plugin_path khop basename) va
truyen vao render_offline khi track khong nhung preset_data.

app/core/native_render.py

@@ -137,6 +137,43 @@ def _preset_b64(preset_id, preset_path, preset_data_b64) -> str:
     return ""
 
 
+def bridge_state_preset_b64(channel=None, plugin_path=None) -> str:
+    """presetBase64 từ bridge_state.json — bridge C++ đang giữ preset user
+    chỉnh realtime (capture mỗi ~2s). Export tạo bridge mới không có preset →
+    plugin load init patch → âm khác realtime (user tưởng "fallback"). Fallback
+    này nối: dùng preset mà bridge đã capture cho channel đó.
+
+    Match: channel == int(track.id) (scheduleMidiNoteDispatch pushEvent
+    channel: trkId) + path khớp plugin (basename, case-insensitive). Không tìm
+    thấy → "" (giữ hành vi cũ — không đoán bừa preset channel khác)."""
+    try:
+        st_path = os.path.join(os.environ.get("APPDATA", ""), "SonicForgeDAW",
+                               "state", "bridge_state.json")
+        if not os.path.isfile(st_path):
+            return ""
+        with open(st_path, encoding="utf-8", errors="replace") as fh:
+            st = json.load(fh)
+        try:
+            ch = int(channel) if channel is not None else None
+        except Exception:
+            ch = None
+        want = None
+        if plugin_path:
+            want = os.path.basename(os.fspath(plugin_path)).lower()
+        for ins in st.get("instruments") or []:
+            if ch is not None and ins.get("channel") != ch:
+                continue
+            ip = ins.get("path") or ""
+            if want and os.path.basename(ip).lower() != want:
+                continue
+            pb = ins.get("presetBase64") or ""
+            if pb:
+                return pb
+    except Exception:
+        pass
+    return ""
+
+
 def _run_notes_job(job: dict, label: str) -> tuple:
     """Chạy job MIDI render qua native_bridge --render → (out_path, duration_sec).