fix(server): use headless-safe arena simulation

Game/scripts/networked_match.gd

@@ -435,7 +435,11 @@ func _start_server() -> void:
 	var arena_path := server_arena_override if not server_arena_override.is_empty() else ArenaRegistry.random_path()
 	server_arena_override = ""
 	_arena_path = arena_path
-	arena = (load(arena_path) as PackedScene).instantiate()
+	# The authoritative simulation needs the shared goals, boundary, and spawn
+	# markers, not a renderable arena variant. Some imported decoration scenes
+	# cannot be instantiated by a dedicated export; clients still receive
+	# arena_path below and instantiate that variant for presentation.
+	arena = (load(ScenePaths.SERVER_ARENA) as PackedScene).instantiate()
 	add_child(arena)
 	for goal in arena.get_goals():
 		goal.goal_scored.connect(_handle_goal_scored)

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"