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.

modules/protocol/inspircd/src/lib.rs

@@ -53,7 +53,19 @@ impl Protocol for InspIrcd {
             None => return vec![],
         };
         match cmd.to_ascii_uppercase().as_str() {
-            "SERVER" => vec![NetEvent::Registered],
+            // A sourced SERVER ( :<parent> SERVER <name> <sid> [hops] :<desc> ) is a
+            // downstream link — track it for the server tree. The unsourced auth
+            // handshake ( SERVER <name> <pass> <sid> … ) is our uplink registering.
+            "SERVER" => match source {
+                Some(parent) => {
+                    let _name = tokens.next();
+                    match tokens.next() {
+                        Some(sid) if !sid.is_empty() => vec![NetEvent::ServerLink { sid: sid.to_string(), parent }],
+                        _ => vec![],
+                    }
+                }
+                None => vec![NetEvent::Registered],
+            },
             "ENDBURST" => vec![NetEvent::EndBurst],
             "PING" => {
                 let token = tokens
@@ -483,6 +495,16 @@ mod tests {
         assert!(matches!(ev.as_slice(), [NetEvent::UserKilled { uid }] if uid == "0IRAAAAAB"), "{ev:?}");
     }
 
+    // The unsourced auth handshake registers our uplink; a sourced SERVER is a
+    // downstream link tracked (with its parent) for the server tree.
+    #[test]
+    fn parses_server_link() {
+        let mut p = proto();
+        assert!(matches!(p.parse("SERVER hub.example password 0IR :a hub").as_slice(), [NetEvent::Registered]), "uplink handshake");
+        assert!(matches!(p.parse(":0IR SERVER leaf.example 0XY :a leaf").as_slice(),
+            [NetEvent::ServerLink { sid, parent }] if sid == "0XY" && parent == "0IR"), "downstream link");
+    }
+
     // A SQUIT surfaces as a ServerSplit carrying the departed server's SID.
     #[test]
     fn parses_squit() {