Add headless regression tests for Phase 4b features

- Implement test for popup anchor behavior in rule-builder menus to ensure consistent anchor positioning during menu transitions.
- Create tests for stage logic, including mode transitions, toolbar visibility, and status bar updates.
- Add terrain drag-painting tests to verify correct block placement behavior and conflict handling.
- Introduce walk waypoint tests to check for arrival conditions and position stability after navigation.

scripts/thumbnails/thumbnail_cache.gd

@@ -0,0 +1,58 @@
+class_name ThumbnailCache
+extends RefCounted
+
+const STICKMEN_DIR := "user://thumbnails/stickmen"
+const PROP_DIR := "user://thumbnails/props"
+const PROP_VERSION := 1
+
+
+func stickman_key(path: String) -> String:
+	return "%s_%d" % [path.get_file().get_basename(), FileAccess.get_modified_time(path)]
+
+
+func stickman_png(key: String) -> String:
+	return STICKMEN_DIR + "/" + key + ".png"
+
+
+func prop_png(id: String) -> String:
+	return PROP_DIR + "/" + id + "_v" + str(PROP_VERSION) + ".png"
+
+
+func load_png(png_path: String) -> Texture2D:
+	if not FileAccess.file_exists(png_path):
+		return null
+	var img := Image.load_from_file(png_path)
+	if img == null:
+		return null
+	return ImageTexture.create_from_image(img)
+
+
+func save_png(tex: Texture2D, png_path: String) -> Error:
+	if tex == null:
+		return ERR_INVALID_PARAMETER
+	var img := tex.get_image()
+	if img == null:
+		return ERR_CANT_CREATE
+	ensure_dir(png_path.get_base_dir())
+	return img.save_png(png_path)
+
+
+func ensure_dir(dir: String) -> void:
+	DirAccess.make_dir_recursive_absolute(dir)
+
+
+func clean_stale_stickmen(valid_keys: Dictionary) -> void:
+	if not DirAccess.dir_exists_absolute(STICKMEN_DIR):
+		return
+	var dir := DirAccess.open(STICKMEN_DIR)
+	if dir == null:
+		return
+	dir.list_dir_begin()
+	var fname := dir.get_next()
+	while fname != "":
+		if not dir.current_is_dir() and fname.ends_with(".png"):
+			var key := fname.trim_suffix(".png")
+			if not valid_keys.has(key):
+				DirAccess.remove_absolute(STICKMEN_DIR + "/" + fname)
+		fname = dir.get_next()
+	dir.list_dir_end()