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.

docs/specs/phase_19_scenery_backgrounds_ui_ux.md

@@ -118,8 +118,8 @@ These are relationship examples, not fixed pixel geometry. On a narrow viewport
 
 ### Wall panel
 
-- **Backgrounds → Wall panel** offers picture choices **Paint**, **Brick**, and **Wood**, plus a large **Color** swatch/button. Common colors appear first; **More colors…** opens a themed color dialog with a visible Cancel. Finish and color choices are reflected in a small preview and then in the translucent Stage draft.
-- Choosing the look closes the chooser and arms **Draw wall panel**. The capture bar says **“Drag across the space for a wall panel”**, with **Done** and **✕ Cancel**. The pointer has a corner reticle and a nearby pill **“Drag to draw wall”**. A press sets the first corner; dragging draws a world-anchored translucent patterned rectangle; release commits one finite panel. The pattern preview remains at sensible scale rather than stretching with the draft.
+- **Backgrounds → Wall panel** offers picture choices **Paint**, **Brick**, and **Wood**, plus a large preview of only the selected finish (Paint is a flat color; Brick and Wood show their texture). The selected-color tile replaces the chooser content and footer with the themed custom color view; **Back to wall panel** discards its draft and returns to setup, while **Use color** applies it and returns. The chooser's **Recently used** row is the only recent-color list. In the expanded setup chooser, **Cancel** exits and **Draw wall panel** starts placement; there is no separate Back or close-X action in that state.
+- Choosing a look updates its preview. Pressing **Draw wall panel** closes the chooser and arms placement. The capture bar says **“Drag across the space for a wall panel”**, with **Done** and **✕ Cancel**. The pointer has a corner reticle and a nearby pill **“Drag to draw wall”**. A press sets the first corner; dragging draws a world-anchored translucent patterned rectangle; release commits one finite panel. The pattern preview remains at sensible scale rather than stretching with the draft.
 - Snap, when enabled, visibly aligns the draft to nearby grid lines. An unusably small draft shows a red outline and **“Make the wall bigger”**; release leaves no panel. A valid draft uses the EDIT accent plus a neutral outline and dimension cue. This is appearance only: a helper in the chooser says **“Looks like a wall. Stickmen can pass through.”**
 - Touch can drag or tap the first and second corners. The first tap leaves a visible anchored corner and continuous rectangle preview; the second tap commits. The same visible Cancel and Done remain available. Right-click cancels a draft and exits the wall tool under the exit rules below.
 
@@ -219,7 +219,7 @@ This matrix identifies current UI that already exists. “Extend/share” is a d
 | Scenery chooser and Artwork cards | `AssetSelector`, `AssetCard`, `StageAssetBrowser`, thumbnail cache/renderers, `scenes/stage/asset_selector.tscn` | **Extend/share the gallery shell and card behavior, reuse browsing/thumbnail work.** The current selector is a fixed four-column paged popup and sets card focus to `FOCUS_NONE`; these must be corrected for responsive layout and keyboard use in the common path rather than copied into a Scenery-only popup. Backgrounds/Artwork tabs use native button choice states and shared popup styling; placement-role text stays feature-local. |
 | Sky and wall/Ground finish cards | `AssetCard` and the responsive gallery behavior above; `SandboxTheme` popup/card tokens | **Share the same preview-card and focus/selected-state rules.** Sky and finishes supply their own preview imagery and one-tap selection meaning; card dragging is disabled for these choices. They do not pretend to be `.prop` assets or reuse `StageAssetBrowser` asset scanning. Wall and Ground share names/preview motifs for Paint, Brick, and Wood. |
 | Artwork tags, Props filters, authoring tags | No existing reusable tag-chip control; `StagePanelChoiceGroup` is a vertical exclusive inspector control; Stage speech UI demonstrates `HFlowContainer` wrapping | **Extract one shared wrapping filter/choice treatment.** Use native focusable buttons and one theme contract across Scenery browsing, Props browsing, and optional multi-tag authoring. Single-choice browser filters and multi-choice authoring use the same visual states but different selection rules. Do not clone local chip styling or force the vertical inspector group into a wrapping browser row. |
-| Wall Color and More colors dialog | `ColorSwatchSelector`; native `ColorPickerButton` already used by the Prop Editor; `StageModalViews` dialog theme | **Extend/share the color-swatch treatment** so it wraps on narrow screens, has visible chosen/focus states, and uses Stage tokens. Use native color picker behavior in a themed Stage dialog. The Prop Editor's `PropTheme` styling is not a Stage style source; wall/color values remain feature-local. |
+| Wall Color and custom color view | `ColorSwatchSelector`; native `ColorPicker` in the themed Stage view | **Extend/share the color-swatch treatment** so it wraps on narrow screens, has visible chosen/focus states, and uses Stage tokens. Open custom color selection from the selected-color tile, keep one recent-color row in the chooser, and provide **Back to wall panel** / **Use color** actions. The Prop Editor's `PropTheme` styling is not a Stage style source; wall/color values remain feature-local. |
 | Scenery overlap **Pieces here** | `StageModalViews.show_objects_here()` and its themed, viewport-clamped, scrollable `PopupPanel` | **Generalize the existing overlap chooser.** Supply Scenery-only candidates and child-facing piece names/depth, while retaining the same placement, 44 px rows, dismiss, and focus behavior. Do not create a second overlapping-items popup. |
 | Missing Scenery artwork | Existing `MissingPropPlaceholder`/`MissingStickmanPlaceholder` preserve missing placements; `StageModalViews` missing-assets dialog; `StageInspectorPanel`; `StageAssetBrowser` gallery/file chooser | **Reuse and generalize the recovery path.** Keep saved placement continuity and one themed load summary, but give Scenery its neutral labeled marker, selection-card repair actions, and the existing artwork/file choosers. Do not copy the older red marker styling or create a second asset gallery or dialog skin. |
 | **Erase this piece?** | `StageModalViews` confirmation/dialog theming; existing Erase Use spot `ConfirmationDialog` and safe/danger treatment in `SandboxStage` | **Reuse the confirmation pattern.** Share the button sizing, themed window, and danger/safe styles instead of copying the spot-erase styling into another private function. The affected piece and wording are local. |