Proactive commands with a body over 127 bytes (e.g. SET UP MENU with a
long item list) use a long-form length (0x81 NN / 0x82 NN NN) in their
D0 wrapper. Previously parse_tlv bailed on those, so the FETCH decoded
as a bare 'FETCH' without its command name or items.
Tests: 105 (long-form 1/2-byte, real SET UP MENU sample).