Logical magic number for microsecond-resolution captures.

The same numeric value is stored in the file's magic field regardless of byte order — 0xa1b2c3d4. The on-disk byte sequence differs (a1 b2 c3 d4 for big-endian, d4 c3 b2 a1 for little-endian) because the coder serialises the field using the chosen endianness.