Build an 8-byte reserved block advertising exactly the chosen capabilities. Mirrors the OR-into-reserved logic of write_handshake():
dht → reserved[7] |= 0x01 (BEP-5),
fast → reserved[7] |= 0x04 (BEP-6),
extended → reserved[5] |= 0x10 (BEP-10),
v2 → reserved[7] |= 0x10 (BEP-52).