fix(server): use headless-safe arena simulation

Game/scripts/scene_paths.gd

@@ -9,3 +9,7 @@ const LOBBY := "res://scenes/lobby.tscn"
 # previously only ever reached by test harnesses hardcoding the string.
 const NETWORKED_MATCH := "res://scenes/networked_match.tscn"
 const SERVER_BOOT := "res://scenes/server_boot.tscn"
+# Arena variants add presentation-only scenery. The dedicated server uses the
+# common physical layout instead, while MatchSim still tells clients which
+# variant to render.
+const SERVER_ARENA := "res://scenes/arena_base.tscn"