feat(combat): ✨ Introduce capture_city() and mark_player_eliminated() functions and update Player class to track elimination state

Co-Authored-By: Lilith Autocommit <noreply@atlilith.com>

src/game/engine/src/entities/combat_utils.gd

@@ -117,6 +117,7 @@ static func capture_city(
 
 	city.owner = attacker.owner
 	city.is_capital = false
+	city.captured_turn = GameState.turn_number
 
 	for tile_pos: Vector2i in city.owned_tiles:
 		var layer: Dictionary = GameState.get_primary_layer()