The two 0xff bytes were intended to easily distinguish files with
whole file signatures from those without, but I got the endianness
backwards. Go ahead and fix that, as long as I'm making changes to
the verifier anyway.
Check for a signature that includes the sequence 0x50 0x4b 0x05 0x06,
which looks to minzip like the start of the EOCD block.
badd2ca451