feat(items): add ChestItemData resource and RoomChestConfig static config

TDD: 4 tests written first (FAIL), then implemented — all 151 tests pass.

test/unit/test_room_chest.gd

@@ -0,0 +1,22 @@
+## Tests for ChestItemData resource and RoomChestConfig static config.
+extends GutTest
+
+
+func test_chest_item_data_default_item_type_is_holdable() -> void:
+	var d: ChestItemData = ChestItemData.new()
+	assert_eq(d.item_type, "holdable")
+
+
+func test_chest_item_data_default_outfit_layer_is_one() -> void:
+	var d: ChestItemData = ChestItemData.new()
+	assert_eq(d.outfit_layer, 1)
+
+
+func test_room_chest_config_reception_desk_has_three_items() -> void:
+	var items: Array[ChestItemData] = RoomChestConfig.get_items("reception_desk")
+	assert_eq(items.size(), 3)
+
+
+func test_room_chest_config_unknown_id_returns_empty() -> void:
+	var items: Array[ChestItemData] = RoomChestConfig.get_items("does_not_exist")
+	assert_eq(items.size(), 0)