Add Phase 19 Starter Scene Composition UI/UX Specification and Tests

- Introduced a new UI/UX specification document for the Phase 19 Starter Scene Composition, detailing user paths, composition rules for Town Street and Living Room, camera framing, and observable acceptance outcomes.
- Implemented a visual capture test for different sky states in the sandbox stage, ensuring proper rendering and visual consistency.
- Created a comprehensive test suite for the Phase 19 starter scene composition, validating scene loading, geometry, navigation, and interaction behaviors for both Town Street and Living Room.
- Added checks for visual alignment, object placements, and scene transitions to ensure adherence to the new UI/UX specifications.

docs/specs/phase_19_scenery_backgrounds_ui_ux.md

@@ -1,6 +1,6 @@
 # Phase 19 — Scenery, backgrounds, and surface finishes: UI/UX specification
 
-> **Status:** Approved and implemented, including the Stage-relative asset-sizing addition (2026-10-16). Based on the [accepted product brief](../plans/phase_19_scenery_backgrounds_surface_finishes.md). Full Phase 19 user/editor verification remains pending.
+> **Status:** The original Phase 19 UI/UX is approved and implemented, including the Stage-relative asset-sizing addition (2026-10-16). The accepted camera-aware Sky behavior is being specified as a focused Phase 19 addition. Based on the [accepted product brief](../plans/phase_19_scenery_backgrounds_surface_finishes.md). Full Phase 19 user/editor verification remains pending.
 >
 > **Decision:** Use a responsive Scenery chooser followed by canvas tools. Placement stays active for repeated copies. Right-click exits placement. Scenery artwork keeps live `.prop` references, with a visible, repairable placeholder if a referenced file cannot be found. The Phase 19 addition standardizes new artwork around a standing-stickman size guide and adds single-placement Scenery resizing; direct single-piece transforms commit on release, while the existing group preview keeps its Keep/Cancel boundary. Phase 19 does not add Undo; Stage Undo is deferred to its own future phase and should be recorded in tech debt during architecture planning.
 
@@ -114,7 +114,17 @@ These are relationship examples, not fixed pixel geometry. On a narrow viewport
 
 - **Backgrounds → Sky** opens four designed thumbnail cards: **Clear Day**, **Cloudy Day**, **Sunset**, **Starry Night**. The currently selected card has a strong border, filled selected state, and the text **“Current Sky”**; keyboard focus remains a separate outline.
 - One tap or Enter/Space changes the Sky immediately; the chooser stays open so another card can be tried. Close means “stop choosing”, not “revert”. With no Undo in Phase 19, the child can restore the previous look by choosing its card again. Changing Sky never recolors or tints other pieces. Sky has no Stage selection handles or overlap entry.
-- The thumbnail shows enough clouds, sun/moon, and color variation to predict the Stage result. A chosen Sky is static during PLAY and stable through camera movement.
+- The thumbnail shows enough clouds, sun/moon, and color variation to predict the Stage result. It remains a representative still image; the Stage applies its automatic camera response without asking the child to configure motion.
+
+### Camera-aware Sky details
+
+- The four existing Sky cards and their immediate selection flow stay unchanged. The selected card still says **Current Sky**. Do not add a motion toggle, slider, new label, tutorial cue, cursor, or theme token.
+- Keep the vertical gradient fixed to and filling the full viewport. Clouds, sun, moon, and stars respond automatically and subtly to Stage camera pan and zoom in **EDIT**, **DIRECT**, and **PLAY**. They move less and change size less than the stickman and Ground, so they read as distant details while connecting to a dramatic shot.
+- Preset-specific composition keeps the main action area clear at ordinary framing. During close and wide views, details remain recognizable and do not grow across the character or walkable Ground. Camera response stays continuous and bounded; it does not expose an edge, pop, sweep distractingly, or reverse abruptly.
+- At unchanged camera framing, switching modes, starting/stopping PLAY, and reopening a saved scene reproduce the same Sky composition. On viewport resize, the gradient continues to cover the view and the details recompose coherently.
+- Keep landscape edges on world-anchored **Background ground** and scenery. Sky details do not draw a horizon, gain selection handles, enter selection, or alter the colors or behavior of world pieces.
+- The visible feedback is the Sky details' restrained response to the same pan/zoom gesture already used on the Stage. No additional status message or control feedback is needed.
+- Acceptance path: choose each Sky, then pan and zoom through ordinary, close, and wide framing in EDIT/DIRECT/PLAY; resize the viewport; start/stop PLAY; save/reopen with a saved camera view; and place Background ground behind walkable Ground. Confirm a subtle but perceptible Sky response, clear action area, no edge/flicker, consistent mode transitions, and Ground alignment.
 
 ### Wall panel