fix(ctypes): pin LPMSG across MSG-pump prototypes for Python 3.13

Python 3.13's ctypes tightened argtypes checks and now rejects
mismatched POINTER(MSG) cache entries — each call to POINTER(MSG)
can return a class identity that doesn't match what byref() of an
instance produces, raising "expected LP_MSG instance instead of
pointer to _MSG" inside GetMessageW/TranslateMessage/DispatchMessageW.

Capture POINTER(MSG) once into LPMSG and reuse the same class object
across all three prototypes in both the WindowsShutdownGuard pump and
the PlatformDetector display-power monitor. Restores the 4 failing
win_shutdown tests.

server/src/ledgrab/core/automations/platform_detector.py

@@ -84,6 +84,21 @@ class PlatformDetector:
             ]
             user32.DefWindowProcW.restype = ctypes.c_ssize_t
 
+            # Pin the MSG pointer type so byref(msg) matches the prototype
+            # (Python 3.13 ctypes rejects mismatched POINTER(MSG) caches).
+            LPMSG = ctypes.POINTER(ctypes.wintypes.MSG)
+            user32.GetMessageW.argtypes = [
+                LPMSG,
+                ctypes.wintypes.HWND,
+                ctypes.c_uint,
+                ctypes.c_uint,
+            ]
+            user32.GetMessageW.restype = ctypes.c_int
+            user32.TranslateMessage.argtypes = [LPMSG]
+            user32.TranslateMessage.restype = ctypes.wintypes.BOOL
+            user32.DispatchMessageW.argtypes = [LPMSG]
+            user32.DispatchMessageW.restype = ctypes.c_ssize_t
+
             def wnd_proc(hwnd, msg, wparam, lparam):
                 if msg == WM_POWERBROADCAST and wparam == PBT_POWERSETTINGCHANGE:
                     try:

server/src/ledgrab/utils/win_shutdown.py

@@ -133,18 +133,23 @@ def _bind_winapi() -> None:
     ]
     user32.DefWindowProcW.restype = LRESULT
 
+    # Pin the MSG pointer type once and reuse the same class object on all
+    # three prototypes — Python 3.13 ctypes rejects mismatched POINTER(_MSG)
+    # caches across argtypes, even though they look identical.
+    LPMSG = ctypes.POINTER(_MSG)
+
     user32.GetMessageW.argtypes = [
-        ctypes.POINTER(_MSG),
+        LPMSG,
         wintypes.HWND,
         wintypes.UINT,
         wintypes.UINT,
     ]
     user32.GetMessageW.restype = wintypes.BOOL
 
-    user32.TranslateMessage.argtypes = [ctypes.POINTER(_MSG)]
+    user32.TranslateMessage.argtypes = [LPMSG]
     user32.TranslateMessage.restype = wintypes.BOOL
 
-    user32.DispatchMessageW.argtypes = [ctypes.POINTER(_MSG)]
+    user32.DispatchMessageW.argtypes = [LPMSG]
     user32.DispatchMessageW.restype = LRESULT
 
     user32.PostMessageW.argtypes = [