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.

sandbox_theme.json

@@ -0,0 +1,23 @@
+{
+  "version": "1.0",
+  "fonts": {
+    "ui_font": "",
+    "emoji_font": "",
+    "action_popup_font_size": 24,
+    "action_popup_emoji_size": 22,
+    "assignment_badge_font_size": 20,
+    "assignment_badge_radius": 9,
+    "rule_label_font_size": 16,
+    "status_pill_font_size": 16,
+    "tooltip_font_size": 18
+  },
+  "grid": {
+    "snap_size": 15.0
+  },
+  "mode_colors": {
+    "edit_accent": "#22c6ff",
+    "direct_accent": "#ffb300",
+    "play_accent": "#33dd77",
+    "guide_line": "#22c6ff"
+  }
+}