Add missing stickman placeholder and stage scene I/O functionality

- Implemented `MissingStickmanPlaceholder` class to visually indicate missing stickman assets in the scene.
- Created `StageSceneIO` class for handling JSON serialization and deserialization of stage documents, including encoding and decoding of actions and rules.
- Added tests for vector handling, rule encoding/decoding, safe file writing, root validation, and handling of missing assets.
- Introduced a new stickman asset file `bad.stk` for testing purposes.
- Enhanced test coverage for phase 5 file features, ensuring robust handling of scene data and missing assets.

scripts/missing_stickman_placeholder.gd

@@ -0,0 +1,24 @@
+class_name MissingStickmanPlaceholder
+extends Node2D
+
+var original_entry: Dictionary = {}
+var missing_stk_path: String = ""
+
+const MARKER_SIZE := Vector2(260.0, 52.0)
+
+func configure(entry: Dictionary) -> void:
+	original_entry = entry.duplicate(true)
+	missing_stk_path = String(entry.get("stk_path", ""))
+	queue_redraw()
+
+func get_area_rect() -> Rect2:
+	return Rect2(-MARKER_SIZE * 0.5, MARKER_SIZE)
+
+func _draw() -> void:
+	var rect := get_area_rect()
+	draw_rect(rect, Color(0.76, 0.10, 0.10, 0.95), true)
+	draw_rect(rect, Color(1.0, 0.78, 0.78, 1.0), false, 2.0)
+	var asset_name := missing_stk_path.get_file()
+	if asset_name.is_empty():
+		asset_name = "Unnamed .stk asset"
+	draw_string(ThemeDB.fallback_font, rect.position + Vector2(10.0, 32.0), "Missing asset - %s" % asset_name, HORIZONTAL_ALIGNMENT_CENTER, rect.size.x - 20.0, 16, Color.WHITE)