handshake(sock, our_name=..., cookie=...) drives the OTP 23+ dance over
a 2-byte length-prefixed frame stream:
send_name (N) client -> server
recv_status (s) server -> client
recv_challenge (N) server -> client
challenge_reply (r) client -> server
challenge_ack (a) server -> client
Cookie digest formula md5(cookie ++ integer_to_list(challenge)) was
cross-checked against erlang:md5/1 output as a test reference.
Distribution flags in erldistpy/flags.py advertise the minimum useful
set: extended refs/pids, new fun tags, utf8 atoms, maps, big creation,
v6 handshake, unlink id, v4 node containers.
86 tests green: frame builders + parsers as pure functions, digest
reference, full live handshake against `erl -sname -setcookie`, and a
wrong-cookie rejection test.
Newer SHA-256 digest (DFLAG_MANDATORY_25_DIGEST) deferred until a peer
requires it.
