nickserv: split into one file per command

Mirrors the chanserv layout: nickserv.rs keeps the service and dispatch,
each command (register, identify, logout, cert) lives in its own file
with its subcommands and parameters. Behaviour is unchanged.

modules/nickserv/logout.rs

@@ -0,0 +1,14 @@
+use crate::engine::service::{Sender, ServiceCtx};
+
+// LOGOUT: log out and rename to a guest nick (prefix + a per-logout sequence).
+pub fn handle(me: &str, guest_nick: &str, guest_seq: &mut u32, from: &Sender, ctx: &mut ServiceCtx) {
+    if from.account.is_none() {
+        ctx.notice(me, from.uid, "You're not logged in.");
+        return;
+    }
+    let guest = format!("{guest_nick}{guest_seq}");
+    *guest_seq = guest_seq.wrapping_add(1);
+    ctx.logout(from.uid);
+    ctx.force_nick(from.uid, &guest);
+    ctx.notice(me, from.uid, format!("You're now logged out. Your nick is now \x02{}\x02.", guest));
+}