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/engine/mod.rs

@@ -1,25 +1,21 @@
+pub mod db;
 pub mod service;
 pub mod state;
 
 use crate::proto::{NetAction, NetEvent};
-use service::{Service, ServiceCtx};
-use state::{EventLog, Network};
+use db::Db;
+use service::{Sender, Service, ServiceCtx};
+use state::Network;
 
 pub struct Engine {
     services: Vec<Box<dyn Service>>,
-    #[allow(dead_code)]
     network: Network,
-    #[allow(dead_code)]
-    log: EventLog,
+    db: Db,
 }
 
 impl Engine {
-    pub fn new(services: Vec<Box<dyn Service>>) -> Self {
-        Self {
-            services,
-            network: Network::default(),
-            log: EventLog::default(),
-        }
+    pub fn new(services: Vec<Box<dyn Service>>, db: Db) -> Self {
+        Self { services, network: Network::default(), db }
     }
 
     // Sent right after the SERVER line: burst, introduce every service, endburst.
@@ -41,18 +37,30 @@ impl Engine {
     pub fn handle(&mut self, event: NetEvent) -> Vec<NetAction> {
         match event {
             NetEvent::Ping { token, from } => vec![NetAction::Pong { token, from }],
+            NetEvent::UserConnect { uid, nick } => {
+                self.network.user_connect(uid, nick);
+                Vec::new()
+            }
+            NetEvent::Quit { uid } => {
+                self.network.user_quit(&uid);
+                Vec::new()
+            }
             NetEvent::Privmsg { from, to, text } => self.dispatch(&from, &to, &text),
             _ => Vec::new(),
         }
     }
 
-    // Route a PRIVMSG addressed to a service (by uid or nick) into that service.
+    // Route a PRIVMSG addressed to a service (by uid or nick) into that service,
+    // handing it the sender's resolved nick and the account store.
     fn dispatch(&mut self, from: &str, to: &str, text: &str) -> Vec<NetAction> {
+        let nick = self.network.nick_of(from).unwrap_or(from).to_string();
         let mut ctx = ServiceCtx::default();
-        for svc in self.services.iter_mut() {
+        let sender = Sender { uid: from, nick: &nick };
+        let Self { services, db, .. } = self;
+        for svc in services.iter_mut() {
             if to.eq_ignore_ascii_case(svc.uid()) || to.eq_ignore_ascii_case(svc.nick()) {
                 let args: Vec<&str> = text.split_whitespace().collect();
-                svc.on_command(from, &args, &mut ctx);
+                svc.on_command(&sender, &args, &mut ctx, db);
                 break;
             }
         }