Scaffold the federated services daemon

Protocol-agnostic core: a Protocol trait maps raw server-to-server lines to a
normalized event/action model, so the engine never touches a raw line and a new
ircd is one new module. Adds event-log state (state is a fold over the log) and
a service framework. First protocol impl links to an InspIRCd (insp4) uplink —
completes the CAPAB/SERVER handshake, bursts, and introduces NickServ.

src/engine/state.rs

@@ -0,0 +1,39 @@
+use std::collections::HashMap;
+
+// Live network view the services reason about.
+#[derive(Default)]
+pub struct Network {
+    pub users: HashMap<String, User>,    // keyed by UID
+    pub channels: HashMap<String, Channel>,
+}
+
+pub struct User {
+    pub uid: String,
+    pub nick: String,
+    pub account: Option<String>,
+}
+
+pub struct Channel {
+    pub name: String,
+    pub ts: u64,
+}
+
+// The Sable-inspired core: every persistent change is an Event, and state is a
+// fold over the log. Single-node today; replicating this log across service
+// nodes is what turns it federated later, without rewriting the services.
+#[derive(Debug, Clone)]
+pub enum Event {
+    AccountRegistered { account: String, uid: String },
+    ChannelRegistered { channel: String, founder: String },
+}
+
+#[derive(Default)]
+pub struct EventLog {
+    pub events: Vec<Event>,
+}
+
+impl EventLog {
+    pub fn append(&mut self, event: Event) {
+        self.events.push(event);
+    }
+}