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.

.gitignore

@@ -6,6 +6,8 @@ addons/
 *.tmp
 .opencode/skills/playwright-cli
 *.import
+# Local scratch captures, logs, and temporary browser profiles
+/tmp/
 content/review/artist/
 content/examples/
 /tests/godot_test_user_data/