feat(multiplayer): rotate ranked arenas deterministically

docs/MATCHMAKING.md

@@ -237,7 +237,9 @@ rating loss because no rated match began.
 - Solo queue only at launch.
 - The matcher validates ranked admission against its server-owned allowlist of
   the three floor-goal `ArenaRegistry` entries. Elevated goals remain excluded
-  until the trained-policy restriction is lifted. The selected scene is
+  until the trained-policy restriction is lifted. It chooses from that list
+  deterministically from the proposal ID, so retrying a proposal cannot change
+  its arena. The selected scene is
   persisted with the proposal/match plan, included in the allocation identity,
   and passed through the Agones GameServer annotation into the allocated
   server's validated `--arena-path` flag.