engine: track the server tree so a hub SQUIT cascades

SQUIT arrives once for a splitting server (by SID), but a hub takes its
whole subtree with it — users behind downstream servers were left as stale
state. The parser now distinguishes a sourced downstream SERVER (tracked
with its parent) from the unsourced uplink handshake, the engine keeps the
SID->parent tree, and a SQUIT forgets every user across the departed
server's subtree. Tree is rebuilt each link. Wire formats verified against
the InspIRCd 4 m_spanningtree source.

api/src/lib.rs

@@ -46,8 +46,11 @@ pub enum NetEvent {
     // A user was forcibly removed (KILL). Handled like a quit, except a killed
     // services bot is reintroduced rather than forgotten.
     UserKilled { uid: String },
-    // A server split away (SQUIT). `server` is its SID; every user behind it
-    // (whose uid carries that SID prefix) is gone, since a split is signalled once
+    // A downstream server was introduced (a sourced SERVER). `parent` is the SID
+    // that introduced it, so we can track the tree and cascade a hub's SQUIT.
+    ServerLink { sid: String, parent: String },
+    // A server split away (SQUIT). `server` is its SID; every user behind it — and
+    // behind any server in its subtree — is gone, since a split is signalled once
     // rather than as a QUIT per user.
     ServerSplit { server: String },
     // An ircd relaying an IRCv3 account-registration request to us as the authority.