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/stickman_thumbnail.gd

@@ -0,0 +1,73 @@
+class_name StickmanThumbnail
+extends Node
+
+const STICKMAN_FACTORY := preload("res://scripts/stickman_factory.gd")
+const STAGE_SPAWNER := preload("res://scripts/stage_spawner.gd")
+
+const SIZE := Vector2i(200, 200)
+
+var _viewport: SubViewport
+var _world: Node2D
+var _camera: Camera2D
+
+
+func _ready() -> void:
+	_viewport = SubViewport.new()
+	_viewport.size = SIZE
+	_viewport.transparent_bg = true
+	_viewport.render_target_update_mode = SubViewport.UPDATE_ALWAYS
+	add_child(_viewport)
+	_world = Node2D.new()
+	_viewport.add_child(_world)
+	_camera = Camera2D.new()
+	_camera.enabled = true
+	_viewport.add_child(_camera)
+	_camera.make_current()
+
+
+func render(stk_data: Dictionary) -> Texture2D:
+	_clear_world()
+	var rig: Node2D = STICKMAN_FACTORY.spawn_from_data(stk_data)
+	if rig == null:
+		return null
+	_world.add_child(rig)
+	rig.position = Vector2.ZERO
+	var bbox := STAGE_SPAWNER.get_world_aabb(rig)
+	if not bbox.has_area() or bbox.size.x < 1.0 or bbox.size.y < 1.0:
+		bbox = Rect2(Vector2(-60, -500), Vector2(120, 500))
+	_frame_camera(bbox)
+	await RenderingServer.frame_post_draw
+	await RenderingServer.frame_post_draw
+	var tex := _viewport.get_texture()
+	rig.queue_free()
+	if tex == null:
+		return null
+	var img := tex.get_image()
+	if _is_blank(img):
+		return null
+	return ImageTexture.create_from_image(img)
+
+
+func _clear_world() -> void:
+	for child: Node in _world.get_children():
+		child.queue_free()
+
+
+func _frame_camera(bbox: Rect2) -> void:
+	var margin := 12.0
+	var fit := minf((SIZE.x - margin * 2.0) / bbox.size.x, (SIZE.y - margin * 2.0) / bbox.size.y)
+	_camera.position = bbox.get_center()
+	_camera.zoom = Vector2(maxf(fit, 0.05), maxf(fit, 0.05))
+
+
+func _is_blank(img: Image) -> bool:
+	if img == null or img.is_empty():
+		return true
+	var used := img.get_used_rect()
+	if used.size.x <= 0 or used.size.y <= 0:
+		return true
+	for y: int in range(used.position.y, used.end.y):
+		for x: int in range(used.position.x, used.end.x):
+			if img.get_pixel(x, y).a > 0.0:
+				return false
+	return true