fix: ensure all daw_engine exit on app close (abrupt kill + LAN client open)

- desktop_engine: watchdog parent check via WaitForSingleObject(SYNCHRONIZE)
  instead of OpenProcess — killed parents linger as zombie objects, OpenProcess
  succeeds forever and watchdog never fires; hard-exit after 5s if graceful
  shutdown hangs (open LAN connection)
- lib.rs: Destroyed handler sweeps any orphan daw_engine/daw_vst_bridge by
  image name before exit(0), covering manual-launch and stale engines

desktop_engine.py

@@ -61,12 +61,21 @@ def _parent_alive(pid):
     # pid khong con ton tai.
     if os.name == "nt":
         import ctypes
+        # OpenProcess chi bao NULL khi process object da bi reap (moi handle
+        # dong het). Process vua bi kill (taskkill/crash) con "zombie" mot
+        # thoi gian -> OpenProcess van thanh cong -> watchdog tuong cha con
+        # song -> engine khong bao gio tu thoat. WaitForSingleObject tren
+        # handle co PROCESS_SYNCHRONIZE phan biet dung: object da terminate
+        # thi SIGNALED ngay, du handle con ton tai.
         PROCESS_QUERY_LIMITED_INFORMATION = 0x1000
-        h = ctypes.windll.kernel32.OpenProcess(PROCESS_QUERY_LIMITED_INFORMATION, False, pid)
+        PROCESS_SYNCHRONIZE = 0x00100000
+        h = ctypes.windll.kernel32.OpenProcess(
+            PROCESS_QUERY_LIMITED_INFORMATION | PROCESS_SYNCHRONIZE, False, pid)
         if not h:
-            return False  # tien trinh da chet
+            return False  # process object da mat -> cha da chet
+        terminated = ctypes.windll.kernel32.WaitForSingleObject(h, 0) == 0
         ctypes.windll.kernel32.CloseHandle(h)
-        return True
+        return not terminated
     try:
         os.kill(pid, 0)
         return True
@@ -140,6 +149,17 @@ def main():
                     logging.getLogger("desktop_engine").info(
                         "Parent process exited — shutting down engine")
                     server.should_exit = True
+                    # Graceful shutdown co the TREO vo han: uvicorn
+                    # _wait_tasks_to_complete cho open connection (client LAN
+                    # dang mo browser) va timeout_graceful_shutdown=None.
+                    # => engine song dai khi app chet dot ngot (crash/Task
+                    # Manager). Hard-exit fallback sau 5s dam bao thoat het.
+                    def _force_exit():
+                        time.sleep(5.0)
+                        logging.getLogger("desktop_engine").info(
+                            "Graceful shutdown hung — forcing exit")
+                        os._exit(0)
+                    threading.Thread(target=_force_exit, daemon=True).start()
                     return
         threading.Thread(target=_watchdog, daemon=True).start()