Add tests for scenery surface caching and preview behavior

- Introduced `test_scenery_brick_preview.gd` to validate focused placement and CPU draw-cost for large brick walls.
- Added `test_scenery_surface_cache.gd` to assess texture budget, visual equivalence, and cache invalidation.
- Created `test_scenery_surface_preview.gd` to evaluate adaptive preview selection and placement behavior.
- Implemented various helper functions to measure drawing performance and validate expected behaviors during tests.
- Added unique identifiers for new test files to ensure proper tracking and management.

AGENTS.md

@@ -72,6 +72,25 @@ the configured `run/main_scene`.
   Keep the prompt and visible Cancel together, leave the mode pill unobstructed, and reuse or
   generalize `StageUseTargetCaptureBar` instead of copying its styling.
 
+## Child-facing dialogs and popups
+
+- When a user prefixes a UI/UX question or request with `uiux:` or `uiux,`, route it to the
+  custom UI/UX agent defined in `.codex/agents/uiux.toml`. Treat that prefix as an explicit
+  delegation request. The UI/UX agent is read-only; carry its design guidance into implementation
+  separately when implementation is requested.
+- For new or revised child-facing dialogs, popups, and short setup flows, follow the
+  [Dialog Design Principles](docs/specs/dialog_design_principles.md). Use rounded containers
+  and controls, clear borders, blue primary actions with bold white labels, comfortable button
+  spacing, and large readable type. Prefer a visible, labeled action such as Cancel, Done, or
+  Back; do not make a small corner × the default or only way to close a dialog.
+- When a dialog resizes after a tab, filter, or other in-place choice, preserve its current
+  anchor so the clicked control does not jump away from the pointer. Keep the dialog within the
+  safe viewport by limiting its height and scrolling content; nudge it only as much as needed
+  to keep essential controls visible. Avoid recentering after every content change.
+- Keep reusable values in `SandboxTheme` or `PropTheme`. The Choose Scenery dialog in
+  `scripts/shared/ui/asset_selector.gd` and the `popups.asset_selector` tokens are the current
+  implementation reference. Adapt its layout to each workflow.
+
 ## Required addon
 
 - **Scalable Vector Shapes 2D** (v2.27.7) at `addons/curved_lines_2d/`