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.
alexei.dolgolyov
e4d24a02da