Add StageBackgroundView and related tests for plain background functionality

- Implemented StageBackgroundView class for managing sky and plain backgrounds in the stage.
- Added functionality to set background type and color, bind camera, and apply themes.
- Created test_phase19_plain_background.gd to validate plain background behavior and migration.
- Developed test_phase19_starter_chooser_layout.gd to ensure proper layout and functionality of the starter chooser.
- Included visual capture tests for UI elements and interactions.

docs/specs/phase_19_scenery_backgrounds_ui_ux.md

@@ -96,6 +96,7 @@ These are relationship examples, not fixed pixel geometry. On a narrow viewport
 ### Stage palette and Scenery chooser
 
 - Add one illustrated **Scenery** card to the existing vertically scrollable left palette. Keep **Props** and **Ground** as separate cards, with a short helper line in their chooser/tool: “Stays put”, “Can be part of the action”, or “Stickmen walk here.” The palette card opens the Scenery chooser rather than directly arming a tool.
+- Keep the Scenery card pressed while its chooser and nested views are open. Back navigation keeps it pressed; closing or canceling the full chooser clears it. If a choice starts drawing, show that active state with the existing on-Stage capture bar instead of leaving the chooser card pressed.
 - The chooser has a fixed title/Close row; **Backgrounds** and **Artwork** tabs; a scrollable body; and a fixed lower **Make artwork…** action when relevant. It is a chooser workflow, not a selected-object inspector. It receives `SandboxTheme.control_theme()` and its own popup tokens, while borrowing the Stage panel system's font, surface, selected, hover, focus, and target-size rules.
 - **Backgrounds** shows two large category choices, **Sky** and **Wall panel**. **Artwork** shows a single-select filter row with **All artwork**, **Nature**, **Buildings**, **Walls**, **Roads & paths**, **Objects**, and conditional **Unsorted**. One drawing may carry several tags, so it appears under every matching filter. All artwork includes old, untagged drawings. Filters describe appearance, not behavior. Apply the same filters in the Props browser while keeping its title and placement role distinct.
 - Use a `FlowContainer` for wrapping filter buttons and an adaptive card grid for previews. Each card has a large illustration, a readable name, and a visible keyboard focus ring. No card depends on hover to reveal its action. Changing tabs or tags preserves the chooser and moves focus to the first relevant control; closing returns focus to the Scenery palette card.
@@ -285,3 +286,4 @@ Do not expose “entity”, “collision hull”, “z-index”, “physics laye
 7. In Living Room, use Select several to choose two wall panels, the window frame, and rug. Preview a shared move, cancel with right-click, and verify all four return exactly to their original locations while Wood Ground and furniture remain still. Preview a uniform corner resize, check the opposite anchor stays fixed and wall motifs do not stretch, then Keep changes; save and reopen. The four pieces retain their new positions/sizes and original depth/finish, while the temporary group selection is gone. Repeat the preview with touch or keyboard controls in a narrow viewport, using Esc to cancel.
 8. Save a scene with two copies of one live `.prop` Scenery drawing, then make the file unavailable and reopen the scene. The friendly missing-art summary appears once; both labeled markers remain at their saved positions/depths through EDIT and PLAY. In ordinary EDIT, select a character in front of a marker without selecting Scenery. Use a marker's Find artwork badge or the Edit Scenery **Needs artwork** list to open its card. Cancel a file choice and verify both markers remain. Choose a replacement for one copy, preview then cancel once, then confirm it; only that copy changes. Confirm the second still has a marker after save/reopen until it too is repaired or deliberately erased. Repeat the repair path with keyboard and on a narrow touch viewport.
 9. In Prop Editor, open **Size for Stage** on representative tall, wide, and small artwork. Confirm the standard stickman is drawn in front, the floor/contact guide is visible over every fill, the preview frames both together, presets and fine adjustments preserve proportions, **Keep size** applies, and **✕ Cancel** restores the exact prior geometry. Confirm new placement uses the authored size and loading an existing file performs no automatic normalization. Deliberately save revised geometry over a referenced asset and confirm the usual Save flow and live-reference update on reopen, with scene placement transforms retained. In Edit Scenery, resize one placement and confirm it commits on release and saves/reopens; cancel a still-active drag with Escape/right-click and with the touch capture bar; verify other scenery and Props stay unchanged. Compare this with the separate group Keep/Cancel flow. Repeat sizing-guide visibility and Stage handle/button access with keyboard/touch and at a narrow viewport.
+10. Open Scenery and navigate between Backgrounds, Plain Background color choices, and its custom picker. Back keeps the Scenery card pressed; Close or Escape from the full chooser clears it. Choose an artwork or surface tool and confirm the chooser card is unpressed while the on-Stage capture bar shows the active tool.