Add Stage Surface Color Dialog and Background Ground Functionality

- Implemented StageSurfaceColorDialog for custom color selection with live preview.
- Created StageBackgroundGround class for non-walkable scenery behind the set.
- Introduced StageRectSurface for shared rectangular geometry and tint preview.
- Added StageSurfacePreview for rendering surface previews.
- Developed comprehensive tests for surface color selection, rendering, and serialization.

docs/architecture/tech_debt_and_optimizations.md

@@ -82,7 +82,9 @@ This document tracks known technical debt, optimization opportunities, and minor
 | 70  | **Impact strength is initially speed based while prop mass is preset based** — material presets set `RigidBody2D.mass`, but fixed props are frozen and mass is not a child-facing setting. | Low | Implemented; future design | Phase 17c keeps the Wobble boolean and visual-only response independent of mass. Impact scoring is centralized in `StageImpactResolver` so a future editable-mass or impulse design can be evaluated without altering the Stage switch or migrating old scene choices. Do not infer a physical impulse from a frozen prop's preset mass; verify any future mass effect separately. See `docs/specs/phase_17c_impact_wobble.md`. (2026-10-14) |
 | 71  | **Palette art is supplied at much higher resolution than its display size** — six 1254 × 1254 PNGs appear in 64 × 64 card image areas. | Low | ✅ Resolved | Phase 18 is implemented and user-verified. The original source PNGs remain 1254 × 1254, Godot imports are capped at 256 px, and `StageChrome` maps the six images by placement ID. The focused palette suite passed; the headless Sandbox Stage loaded, and `git diff --check` passed. Keep the map local to the fixed Stage palette; revisit a data-driven icon descriptor only if tools or themed art become extensible. See `docs/specs/phase_18_palette_icons.md`. (2026-10-14) |
 | 72  | **Stage-wide Undo/Redo needs its own phase** — Phase 19 placement, erase, style, repair, selected-artwork depth, direct single-piece moves/resizes, and group Keep actions change the document without an integrated command history. | Medium | Deferred to separate phase | Design one Stage-level history with coherent commands across Scenery, Ground, Props, terrain, characters, scene naming, and directed content, with safe boundaries around PLAY/paused edits and save/recovery. The Phase 19 sizing amendment keeps single-piece gestures commit-on-release with cancellable active snapshots and retains group Keep/Cancel; neither is an Undo implementation or shortcut. Selected-artwork depth choices also commit immediately. User explicitly deferred Stage Undo/Redo to its own phase. See `docs/specs/phase_19_scenery_backgrounds_surface_finishes.md` and `docs/specs/phase_19_selected_artwork_depth_inspector.md`. (2026-10-17) |
-| 73  | **Whole-set transform requires cross-system invariants** — Phase 19's temporary group transform covers wall panels and Scenery artwork only. | Medium | Future feature | Moving/scaling Ground, characters, interactive Props, Sky, camera composition, waypoints, and rules together needs separate physics, navigation, event-target, and save contracts. Do not expand the Scenery group tool into a whole-set transform without that design. See `docs/specs/phase_19_scenery_backgrounds_surface_finishes.md`. (2026-10-15) |
+| 73  | **Whole-set transform requires cross-system invariants** — Phase 19's temporary group transform covers only inert Scenery pieces: wall panels, artwork, and the planned Background ground. | Medium | Future feature | Moving/scaling walkable Ground, characters, interactive Props, Sky, camera composition, waypoints, and rules together needs separate physics, navigation, event-target, and save contracts. Do not expand the Scenery group tool into a whole-set transform without that design. See `docs/specs/phase_19_scenery_backgrounds_surface_finishes.md` and `docs/specs/phase_19_background_ground_and_wall_color.md`. (2026-10-17) |
+| 74  | **Background surface motifs need a shared zoom-consistent draw budget** — a pattern detail scale derived from each panel's size can shift seams or texture density where adjacent pieces meet, while uncapped large rectangles can issue excessive draw calls. | Medium | Planned | Phase 19 follow-up uses world-cell coordinates, visible-region culling, and any level-of-detail choice derived from camera zoom for all pieces in the same view. Keep Grass/Concrete/Carpet detail deterministic and bounded; consider a texture cache only if measured scenes remain expensive. See `docs/specs/phase_19_background_ground_and_wall_color.md`. (2026-10-17) |
+| 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) |
 
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@@ -174,5 +176,6 @@ This document tracks known technical debt, optimization opportunities, and minor
 | 2026-10-15 | Phase 19 architecture specification drafted for final user sign-off (`docs/specs/phase_19_scenery_backgrounds_surface_finishes.md`) after approval of live `.prop` references with a repairable missing-art placement, adaptive viewport Sky, and a Stage-hosted Prop Editor route. The design adds inert wall/artwork nodes, vector finishes, scene v1.5 migration, `.prop` v1.1 tags, Scenery-only selection and snapshot group transforms, shared Stage UI generalizations, and ordinary editable starter scenes. Logged #72 for a separate Stage Undo/Redo phase and #73 for a separately designed whole-set transform. No runtime code or assets changed. |
 | 2026-10-16 | Added the Phase 19 Stage-relative sizing architecture amendment to `docs/specs/phase_19_scenery_backgrounds_surface_finishes.md` for final sign-off. Prop Editor Size for Stage uses a reversible snapshot, longest-edge presets of 0.35/1/2 standing-stickman heights, origin-based uniform geometry/stroke/editable-anchor scaling, and an editor-only foreground figure/floor comparison. A selected Scenery piece uses its own snapshot transaction and commits on release; a group still uses Keep/Cancel. Existing `.scene` v1.5 placement scale and `.prop` v1.1 geometry need no schema change or load-time normalization. Explicit Save at an existing `.prop` path retains live-reference updates for all users of that source; no asset-version enforcement or new save prompt was requested. Updated #72 for the deferred Stage Undo boundary. The amendment needs no new art and Phase 19 user/editor verification remains pending. No runtime code changed by this specification entry. |
 | 2026-10-17 | Wrote the Phase 19 selected-artwork depth inspector specification (`docs/specs/phase_19_selected_artwork_depth_inspector.md`) for final sign-off. It reuses the existing `StageInspectorPanel` and `StagePanelChoiceGroup`, applies Far/Near/On the floor immediately to one live or missing Scenery placement through `StageSceneryArtwork.set_depth()`, and follows the Stage's existing dirty/save path. The current `.scene` depth field, `StageDrawOrder`, and `stage_panels` tokens need no changes. Updated #72 to include immediate depth edits in a future Stage-wide Undo phase. No runtime code changed by this specification entry. |
+| 2026-10-17 | Wrote the Phase 19 Background ground and wall color architecture specification (`docs/specs/phase_19_background_ground_and_wall_color.md`) for final sign-off. It defines distinct inert wall/backdrop scene types over a small shared rectangle contract, world-aligned bounded vector materials, selected-piece color preview without mutating saved tint, one eight-color Stage settings list, scene v1.6 migration of old Brick/Wood placeholder green, shared Stage inspector controls, and confirmed single-piece backdrop erasure. Logged #74 for zoom-consistent motif budgets and #75 for the old unused wall-color ambiguity. No runtime code changed. |
 
 _End of Log_