Add MoveFileExW mock to system.h/.c

Add mock implementation for MoveFileExW Windows API function following
the existing pattern in the codebase. The mock forwards calls to the
real Windows API, allowing for future interception in the simulation
framework if needed.

src/system.c

@@ -1648,6 +1648,12 @@ BOOL mock_FindClose(HANDLE hFindFile)
     return TRUE;
 }
 
+BOOL mock_MoveFileExW(WCHAR *lpExistingFileName, WCHAR *lpNewFileName, DWORD dwFlags)
+{
+    // Forward to real MoveFileExW, last error is set by the real call
+    return MoveFileExW(lpExistingFileName, lpNewFileName, dwFlags);
+}
+
 #else
 
 int mock_clock_gettime(clockid_t clockid, struct timespec *tp)