Add persisted account store with NickServ REGISTER/IDENTIFY

Accounts are an append-only JSONL event log (state is the fold of the log) with
argon2id-hashed passwords. NickServ gains REGISTER <password> [email] and
IDENTIFY <password>; the engine tracks UID->nick from the burst so a command
resolves to the sender's current nick. Classic and ircd-agnostic — the cap-based
account-registration forward is the next step.

src/proto/mod.rs

@@ -10,6 +10,7 @@ pub enum NetEvent {
     EndBurst,
     Ping { token: String, from: Option<String> },
     Privmsg { from: String, to: String, text: String },
+    UserConnect { uid: String, nick: String },
     Quit { uid: String },
     Unknown { line: String },
 }