fix(local-tool): suppress KneronPLUS DeviceGroup.__del__ access violation

Windows 上 bridge script 結束時 Python GC 呼叫 DeviceGroup.__del__ →
kp_disconnect_devices 對已釋放的 native handle 操作 → OSError: access
violation reading 0x00...0C。這是 KneronPLUS SDK 的 destructor 沒做
null check 的已知問題，不影響功能但會印嚇人的 stack trace 到 stderr。

修法：
- 新增 _cleanup() 函式：明確 kp.core.disconnect_devices + 把
  _device_group 設 None（讓 __del__ 成 no-op）
- atexit.register(_cleanup) 確保 interpreter 關閉前 cleanup
- main() return 後也同步呼叫一次（belt-and-suspenders）

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

local-tool/server/scripts/kneron_bridge.py

@@ -1112,5 +1112,26 @@ def main():
             _respond({"error": str(e)})
 
 
+def _cleanup():
+    """Explicitly disconnect and clear _device_group before Python GC runs.
+
+    KneronPLUS SDK's DeviceGroup.__del__ calls kp_disconnect_devices on a
+    native handle that may already be freed when the interpreter is shutting
+    down, causing 'OSError: access violation reading 0x00...'.  By doing a
+    clean disconnect + setting the global to None here, __del__ becomes a
+    no-op (None has no __del__).
+    """
+    global _device_group
+    if _device_group is not None:
+        try:
+            kp.core.disconnect_devices(_device_group)
+        except Exception:
+            pass
+        _device_group = None
+
+
 if __name__ == "__main__":
+    import atexit
+    atexit.register(_cleanup)
     main()
+    _cleanup()  # also call synchronously in case atexit doesn't fire