Relay: snapshot by_host in the PEER_UP/DOWN fanouts (end-of-round crash)

When every pod disconnects at once (normal end of a multiplayer round),
a failed send inside the PEER_DOWN fanout recursively drops that peer
and mutates by_host mid-iteration -> RuntimeError -> THE RELAY DIES,
taking the round reset and the matchlog uploads with it (live crash,
first 3-player internet round).  Iterate over snapshots.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tools/btconsole.py

@@ -923,7 +923,8 @@ class Relay:
             for other_id in sorted(self.by_host):
                 if other_id != host_id:
                     self._send_control(conn, ROUTE_PEER_UP, other_id)
-            for other in self.by_host.values():
+            for other in list(self.by_host.values()):   # snapshot: a failed
+                # send drops that peer INSIDE the loop (mutates by_host)
                 if other is not conn:
                     self._send_control(other, ROUTE_PEER_UP, host_id)
             return
@@ -990,7 +991,8 @@ class Relay:
         if conn.host_id is not None and self.by_host.get(conn.host_id) is conn:
             del self.by_host[conn.host_id]
             self.udp_endpoint.pop(conn.host_id, None)
-            for other in self.by_host.values():
+            for other in list(self.by_host.values()):   # snapshot: a failed
+                # send drops that peer INSIDE the loop (mutates by_host)
                 self._send_control(other, ROUTE_PEER_DOWN, conn.host_id)
         self._maybe_reset_round()