Credentials: the SCRAM-SHA-256 verifier is now the only password
credential. PLAIN and IDENTIFY verify the plaintext against it via
scram::verify_plain, exactly as a SCRAM login proves knowledge of it, so
an account provisioned from a verifier alone (external authority) works
over every mechanism, not only SCRAM. Removed the redundant argon2 hash
entirely: the field, the Credentials member, the AccountPasswordSet
field, hash_password/verify_password, and the argon2 crate. SASL SCRAM
already forces a PBKDF2 verifier into the store, so the hash never raised
the at-rest floor. OS RNG now comes from rand_core.
Registration: finished draft/account-registration over the ircd relay.
VERIFY confirms the emailed code, RESEND reissues it, STATUS reports
state, and REGISTER now emails the code and replies verification_required
on the relay path too, not only through NickServ.
