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.

master_rig.tscn

@@ -55,10 +55,10 @@ bone_index = 1
 bone2d_node = NodePath("Torso/Head")
 target_nodepath = NodePath("../IK_Targets/Head")
 enable_constraint = true
-constraint_angle_min = 54.99998
-constraint_angle_max = 304.9999
-constraint_angle_invert = true
-constraint_in_localspace = true
+constraint_angle_min = -180
+constraint_angle_max = 180
+constraint_angle_invert = false
+constraint_in_localspace = false
 
 [sub_resource type="SkeletonModificationStack2D" id="SkeletonModificationStack2D_j4hao"]
 modification_count = 5