Add StageBackgroundView and related tests for plain background functionality

- Implemented StageBackgroundView class for managing sky and plain backgrounds in the stage.
- Added functionality to set background type and color, bind camera, and apply themes.
- Created test_phase19_plain_background.gd to validate plain background behavior and migration.
- Developed test_phase19_starter_chooser_layout.gd to ensure proper layout and functionality of the starter chooser.
- Included visual capture tests for UI elements and interactions.

tests/test_phase17b_use_spots.gd

@@ -46,7 +46,7 @@ func _test_migration_and_schema() -> void:
 		var legacy: Dictionary = {"version": version, "objects": [{"file_id": "prop_a", "type": "prop"}]}
 		var migrated: Dictionary = STAGE_SCENE_IO._migrate_to_current(legacy)
 		var entry: Dictionary = migrated["objects"][0]
-		_expect(String(migrated["version"]) == "1.3" and entry.get("use_spots") == [] and int(entry.get("next_use_spot_number", 0)) == 1, "%s prop receives v1.3 empty Use spot defaults" % version)
+		_expect(String(migrated["version"]) == STAGE_SCENE_IO.FILE_VERSION and entry.get("use_spots") == [] and int(entry.get("next_use_spot_number", 0)) == 1, "%s prop receives current-format empty Use spot defaults" % version)
 	var valid_root: Dictionary = _valid_root()
 	_expect(bool(STAGE_SCENE_IO._validate_root(valid_root).get("ok", false)), "valid Use spot data passes root validation")
 	var duplicate: Dictionary = _valid_root()
@@ -176,7 +176,10 @@ func _on_use_failed(reason_code: String, _prop: PropBlock, _spot_id: String) ->
 
 func _valid_root() -> Dictionary:
 	return {
-		"version": "1.3",
+		"version": STAGE_SCENE_IO.FILE_VERSION,
+		"sky_id": "sky_neutral",
+		"background_kind": "sky",
+		"plain_color": STAGE_SCENE_IO.DEFAULT_PLAIN_COLOR,
 		"objects": [{
 			"file_id": "prop_file",
 			"type": "prop",