Fix simulation_server_stub Entity class conflict + ServerConfig load timing

- Renamed inner class Entity→StubEntity (Godot has native Entity class)
- Added await ServerConfig.config_loaded before reading map_list in server_main.gd
- Config loads via call_deferred, so _ready() must wait for signal

server/scripts/simulation_server_stub.gd

@@ -7,9 +7,9 @@
 extends RefCounted
 
 # ---------------------------------------------------------------------------
-# Nested: Entity state data
+# Nested: StubEntity state data
 # ---------------------------------------------------------------------------
-class Entity:
+class StubEntity:
 	var id: int = -1
 	var peer_id: int = -1
 	var position: Vector3 = Vector3.ZERO
@@ -30,7 +30,7 @@ var _movement_config: Dictionary = {}
 var _weapon_config: Dictionary = {}
 var _history_depth: int = 64
 var _next_entity_id: int = 1
-var _entities: Dictionary = {}  # entity_id → Entity
+var _entities: Dictionary = {}  # entity_id → StubEntity
 
 # ---------------------------------------------------------------------------
 # SimulationServer API
@@ -51,7 +51,7 @@ func set_history_depth(depth: int) -> void:
 	_history_depth = depth
 
 func apply_input(entity_id: int, input_dict: Dictionary) -> void:
-	var e: Entity = _entities.get(entity_id)
+	var e: StubEntity = _entities.get(entity_id)
 	if e == null:
 		return
 	# Apply movement from input (simplified)
@@ -72,11 +72,11 @@ func fire_weapon(entity_id: int) -> void:
 	# Stub: log the event
 	pass
 
-func get_entity(entity_id: int) -> Entity:
+func get_entity(entity_id: int) -> StubEntity:
 	return _entities.get(entity_id)
 
 func spawn_player_entity(peer_id: int, pos: Vector3) -> int:
-	var e = Entity.new()
+	var e = StubEntity.new()
 	e.id = _next_entity_id
 	e.peer_id = peer_id
 	e.position = pos