fix(local-tool): DeviceGroup.__del__ access violation 全面修復

上一個 commit (a6a121a) 只修了 script 結束時的 cleanup，但使用者仍在
connect 重試路徑看到 access violation：

  connect attempt 1 failed → 新 connect attempt 2 → GC 回收 attempt 1
  的舊 DeviceGroup → __del__ → kp_disconnect_devices 對已失效的 native
  handle → OSError: access violation

根因：`_device_group = None` 只是清掉 Python reference，舊物件的 __del__
會延遲到下一次 GC cycle（可能發生在新 connect call 的 allocation 時），
此時 native handle 已 invalid。

修法：
- 新增 `_clear_device_group()` helper：先 kp.core.disconnect_devices 把
  native handle 正常釋放（errors silenced），再設 None
- 全檔搜 `_device_group = None` 共 12 處，除了初始宣告（L40）和兩個 helper
  自身（_clear_device_group / _cleanup）以外全部替換為 _clear_device_group()
- 涵蓋所有 code path：connect retry / firmware load reconnect / disconnect
  handler / reset handler / error fallback

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

local-tool/server/scripts/kneron_bridge.py

@@ -38,6 +38,24 @@ except ImportError:
 
 # ── Global state ──────────────────────────────────────────────────────
 _device_group = None
+
+
+def _clear_device_group():
+    """Safely disconnect and clear the global _device_group.
+
+    KneronPLUS SDK's DeviceGroup.__del__ calls kp_disconnect_devices on the
+    native handle, but if the handle is already invalid (failed connect / stale
+    state) it causes 'OSError: access violation'.  By explicitly disconnecting
+    before setting None, __del__ becomes a no-op on an already-disconnected
+    handle.  All errors are silenced — this is best-effort cleanup.
+    """
+    global _device_group
+    if _device_group is not None:
+        try:
+            kp.core.disconnect_devices(_device_group)
+        except Exception:
+            pass
+        _device_group = None
 _model_id = None
 _model_nef = None
 _model_input_size = 224     # updated on model load
@@ -732,7 +750,7 @@ def handle_connect(params):
                 _log(f"KL720: Reconnected after firmware load, pid=0x{target_dev.product_id:04X}, fw={fw_str}")
             else:
                 _log("WARNING: KL720 firmware files not found. Cannot operate with KDP legacy device.")
-                _device_group = None
+                _clear_device_group()
                 return {"error": "KL720 has legacy KDP firmware but KDP2 firmware files not found. "
                         "Run update_kl720_firmware.py to flash KDP2 permanently."}
 
@@ -756,9 +774,8 @@ def handle_connect(params):
         last_err = None
         for attempt in range(max_retries):
             try:
-                # Clear any stale device group from previous failed attempt
-                # to prevent DeviceGroup.__del__ access violation during GC.
-                _device_group = None
+                # Clear any stale device group from previous failed attempt.
+                _clear_device_group()
 
                 if use_without_check:
                     _log(f"{_device_chip}: connect_devices_without_check(usb_port_id={target_dev.usb_port_id}, connectable={target_dev.is_connectable}) attempt {attempt+1}/{max_retries}...")
@@ -774,7 +791,7 @@ def handle_connect(params):
                 last_err = None
                 break
             except Exception as conn_err:
-                _device_group = None  # prevent __del__ crash on stale handle
+                _clear_device_group()
                 last_err = conn_err
                 _log(f"connect attempt {attempt+1} failed: {conn_err}")
                 if attempt < max_retries - 1:
@@ -822,7 +839,7 @@ def handle_connect(params):
                 time.sleep(5)
 
                 # Reconnect after firmware load (with retry)
-                _device_group = None
+                _clear_device_group()
                 for retry in range(3):
                     try:
                         descs = kp.core.scan_devices()
@@ -861,7 +878,7 @@ def handle_connect(params):
         }
 
     except Exception as e:
-        _device_group = None
+        _clear_device_group()
         return {"error": str(e)}
 
 
@@ -870,7 +887,7 @@ def handle_disconnect(params):
     global _device_group, _model_id, _model_nef, _firmware_loaded
     global _model_type, _model_input_size, _device_chip
 
-    _device_group = None
+    _clear_device_group()
     _model_id = None
     _model_nef = None
     _model_type = "tiny_yolov3"
@@ -906,7 +923,7 @@ def handle_reset(params):
         # Even if it throws, the device usually does reset.
 
     # Clear all state — the device is gone until it re-enumerates.
-    _device_group = None
+    _clear_device_group()
     _model_id = None
     _model_nef = None
     _model_type = "tiny_yolov3"