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/stage_selection.gd

@@ -59,6 +59,15 @@ func clear_selection() -> void:
 	selection_changed.emit(_selected.duplicate())
 
 
+## Clears the hovered node (e.g. when it is about to be deleted), emitting
+## hover_changed(null) so the gizmo layer drops its stale highlight.
+func clear_hover() -> void:
+	if _hovered == null:
+		return
+	_hovered = null
+	hover_changed.emit(null)
+
+
 func select_only(node: Node2D) -> void:
 	_selected = [node]
 	_primary = node