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/architecture/tech_debt_and_optimizations.md

@@ -87,6 +87,7 @@ This document tracks known technical debt, optimization opportunities, and minor
 | 75  | **Old Brick/Wood wall colors were saved but not rendered** — v1.5 walls can carry the old default green while their brick and wood palettes were hard-coded. | Low | Planned migration | Scene v1.6 migration replaces only the old green placeholder on pre-v1.6 Brick/Wood entries with natural finish defaults; Paint and non-placeholder colors remain. The exact placeholder could have been intentionally authored in an older file, which cannot be distinguished retrospectively. New v1.6 colors are authoritative, including green Brick/Wood. See `docs/specs/phase_19_background_ground_and_wall_color.md`. (2026-10-17) |
 | 76  | **Camera-aware Sky redraws vector motifs while framing changes** — the former static Sky drew motifs only when the preset or viewport changed. | Low | Planned | The Phase 19 Sky addition keeps one screen-space `StageSkyView`, redraws only when a bound camera's effective framing changes, and caches reusable cloud/moon curve samples where practical. Measure sustained pan/zoom cost in a rendered Stage before adding a raster or shader path. The fixed reference frame and bounded response can saturate at distant camera positions; that is an accepted composition trade-off, not scene state to persist. See `docs/specs/phase_19_scenery_backgrounds_surface_finishes.md#phase-19-addition-camera-aware-sky-details`. (2026-10-21) |
 | 77  | **Starter surfaces guarantee a finite pan envelope, not arbitrary camera framing** — the two bundled sets use a fixed authored width to keep their scene data simple. | Low | Implemented; visual review pending | The authored envelope is a 1,280-logical-pixel viewport at zoom 0.5, ±1,000 world units of travel, and 100 units of margin per side. User review prompted an opaque Town Grass Background ground and separate scale-1 tile rows. Twenty-five 192-unit tiles cover the 4,760-unit minimum; Asphalt has 27 tiles (5,184 units) so the road extends one tile beyond the sidewalk at both ends. Navigation merges contiguous tile tops and routes the 32-unit curb step at the exposed ends without merging Ground objects or colliders. The focused suite passes 515 assertions, including the curb step and PLAY landing. The camera remains unrestricted, so wider viewports, farther pans, and zoomed-out shots can reveal finite ends. Rendered seam review remains pending. See docs/specs/phase_19_starter_scene_composition.md. (2026-10-21)
+| 78  | **Plain Background adds a second backdrop kind while Sky remains a separate renderer** — live Stage, starter previews, and chooser cards must agree on kind and remembered color. | Medium | Spec prepared; implementation pending | Phase 19 composes one shared `StageBackgroundView` from the existing Sky view and an opaque fill. Scene v1.7 stores active kind, last Sky ID, and confirmed plain color; all older scenes migrate as Sky. `SandboxStage` owns the transient custom-picker draft, which must never enter Save or autosave. The existing chooser's four-Sky layout and 42 px compact swatches need targeted extension, and grid/gizmo contrast needs theme-backed treatment for light and dark fills. Rendered pan/zoom/resize, legacy migration, and every draft-cancel exit remain verification gates. See `docs/specs/phase_19_plain_background.md`. (2026-10-22) |
 
 ---
 
@@ -188,4 +189,6 @@ This document tracks known technical debt, optimization opportunities, and minor
 | 2026-10-21 | Implemented the approved starter-scene composition spec. Town Street and Living Room now use continuous 4,760-unit support surfaces, aligned bases, and camera-derived starter-card framing; both edited scene files use current v1.6. The focused production-load/save-reopen/PLAY composition test passed 177 assertions, with measured visible-contact geometry within 0.225 logical pixels. The broader scenery suite exits 1 on nine other background-card and wall-erasure/capture-bar assertions, while the new surface assertions pass. GUI capture exited 134 and headless capture produced no image, so rendered-pixel checks, pan-endpoint seam inspection, narrow viewport/zoom-in framing, and user/editor verification remain pending. Updated #77 from planned to implemented with the finite-camera limitation accepted. See `docs/specs/phase_19_starter_scene_composition.md`.
 | 2026-10-21 | Revised the starters after the user's Town Street screenshot exposed Sky clouds below the skyline and the user rejected oversized scaled Ground blocks. Added an opaque Grass Background ground, replaced the sidewalk and both starter floors with scale-1 tile rows, and extended Asphalt by one tile at each end so the road is accessible. The focused production-path test passed 515 assertions, including navigation across tile seams, sidewalk-to-road routing, and a PLAY actor landing grounded on Asphalt. The broader scenery suite still exits 1 on the same nine Sky-preview/gallery and Scenery-edit UI assertions observed before this revision; its new surface assertions pass. A bounded GUI capture exited 134 without an image, so rendered pixels, pan endpoints, card previews, narrow viewport/zoom-in states, and user/editor review remain pending. See `docs/specs/phase_19_starter_scene_composition.md`.
 
+| 2026-10-22 | Prepared and revised the Phase 19 Plain Background architecture specification (`docs/specs/phase_19_plain_background.md`) for final user sign-off. The accepted composed Stage backdrop view serves live Stage and previews; scene v1.7 persists active kind, last Sky ID, and confirmed plain color while migrating all older scenes as Sky. Color drafts stay out of Save/autosave, and theme-backed grid and two-tone gizmo overlays address light fills without changing art. Per user feedback, the current-color chip is the sole custom-picker entry, the separate More colors button is removed, and the custom picker uses a simple Preview label without a temporary-state helper message. Logged #78 for the chooser extension and rendered verification gates. No runtime code or tests changed. |
+
 _End of Log_