defaultReserved

Build the 8-byte reserved block libtorrent emits by default: DHT + Fast + Extension-protocol bits set (matching write_handshake() with all features compiled in, minus the v2-upgrade bit which depends on torrent state).

v2 adds the BEP-52 v2-upgrade bit (reserved[7] |= 0x10). libtorrent only sets it for a v1 peer in a hybrid torrent (!protocol_v2 && info_hash.has_v2()), so it defaults to false to preserve the prior wire output.