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.

scenes/asset_selector.tscn

@@ -0,0 +1,81 @@
+[gd_scene load_steps=2 format=3]
+
+[ext_resource type="Script" path="res://scripts/asset_selector.gd" id="1_selector"]
+
+[node name="AssetSelector" type="PopupPanel"]
+title = "Asset Selector"
+size = Vector2i(660, 560)
+script = ExtResource("1_selector")
+
+[node name="MarginContainer" type="MarginContainer" parent="."]
+layout_mode = 1
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+grow_horizontal = 2
+grow_vertical = 2
+
+[node name="VBoxContainer" type="VBoxContainer" parent="MarginContainer"]
+layout_mode = 2
+theme_override_constants/separation = 8
+
+[node name="TitleBar" type="HBoxContainer" parent="MarginContainer/VBoxContainer"]
+layout_mode = 2
+
+[node name="TitleLabel" type="Label" parent="MarginContainer/VBoxContainer/TitleBar"]
+unique_name_in_owner = true
+layout_mode = 2
+size_flags_horizontal = 3
+theme_override_font_sizes/font_size = 18
+text = "Choose"
+
+[node name="CloseButton" type="Button" parent="MarginContainer/VBoxContainer/TitleBar"]
+unique_name_in_owner = true
+layout_mode = 2
+text = "×"
+
+[node name="GridContainer" type="GridContainer" parent="MarginContainer/VBoxContainer"]
+unique_name_in_owner = true
+layout_mode = 2
+size_flags_vertical = 3
+theme_override_constants/h_separation = 8
+theme_override_constants/v_separation = 8
+columns = 4
+
+[node name="EmptyLabel" type="Label" parent="MarginContainer/VBoxContainer"]
+unique_name_in_owner = true
+layout_mode = 2
+visible = false
+size_flags_vertical = 3
+text = "No stickmen found! Create one in the editor first."
+horizontal_alignment = 1
+vertical_alignment = 1
+
+[node name="Footer" type="HBoxContainer" parent="MarginContainer/VBoxContainer"]
+layout_mode = 2
+alignment = 1
+
+[node name="PrevButton" type="Button" parent="MarginContainer/VBoxContainer/Footer"]
+unique_name_in_owner = true
+layout_mode = 2
+text = "← Prev"
+
+[node name="PageLabel" type="Label" parent="MarginContainer/VBoxContainer/Footer"]
+unique_name_in_owner = true
+layout_mode = 2
+text = "Page 1/1"
+
+[node name="NextButton" type="Button" parent="MarginContainer/VBoxContainer/Footer"]
+unique_name_in_owner = true
+layout_mode = 2
+text = "Next →"
+
+[node name="BrowseButton" type="Button" parent="MarginContainer/VBoxContainer/Footer"]
+unique_name_in_owner = true
+layout_mode = 2
+text = "Browse…"
+
+[node name="RefreshButton" type="Button" parent="MarginContainer/VBoxContainer/Footer"]
+unique_name_in_owner = true
+layout_mode = 2
+text = "Refresh"