Adding a scene layer

Decisions behind this: ADR 0024 (tiles and their dressing), ADR 0027 (node materials), ADR 0030 (parts on the ground), ADR 0047 (memory). Deep reference: the city-walker skill, "Node materials" and "Tiles".

Decide where it is built

• Depends only on baked data (no look control, no time of day, no device)? Bake it into the fine terrain glTF as a named node in scripts/bake-tiles.ts; the layer file only gives it a material.
• Otherwise build it at runtime in the dressing plugin (tile-stream.ts), as a builder of steps run in slices (lib/city/build-steps.ts) if it can take more than a few ms on a phone.

Checklist

1. Data: a baked per-tile file (new-bake-step skill), declared in lib/city/tile.ts; a missing optional file means "layer off".
2. Frame: positions in the Y-up frame, never inside the rotated world group; heights from the site's ground (lib/city/ground.ts).
3. Material: a node material; a sceneMaterial when it has no per-tile data. Fixed strengths are named constant nodes; anything that moves a shared uniform. Mind the eight vertex buffers and sixteen varyings.
4. Instancing: Instances, never InstancedMesh; chunk into cells so off-screen chunks cull; after writing matrices set needsUpdate and computeBoundingSphere().
5. Lifecycle: build in processTileModel, compile before it shows (PostStack.compile), free in disposeTile. Add its bytes to calculateBytesUsed; add CPU arrays nothing reads later to the CPU-drop lists. A part built after its tile (on demand, or from a file of its own) is a LatePart adapter (late-parts.ts, beside dressingPart and muralsPart): the module builds, weighs, compiles, hangs and frees it; choose hangAt — "wait" only for what a level's hand-over waits on (the dressing), "compile" for everything else.
6. Shadows: casts only if static; call invalidateShadows() when it lands; sway casts through a rigid castShadowPositionNode.
7. Ground: feet and edges from ground-join.ts, joins tested (JOIN_PARTS or checkJoins); its doc comment says what happens where the ground beside it is higher and where it is lower.
8. Night: emissive or additive terms scale by sceneNight (scene-environment.ts); a part drawn only after dark wears drawnAtNight on its scene-wide material; a part whose instances follow the date follows the environment at birth (followEnvironment) and lets go in disposeDressing.
9. Styles: tag what a picture style may dress (userData.styleCrown, userData.styleLampHeads); never read the style.
10. Asking: if a visitor may click it, a ray target from its data (lib/city/ask-items.ts), card lines and a heading in lib/city/inquiry-features.ts, a row in the kind table (lib/city/inquiry.ts), an arm in lineage.ts and a fixture in inquiry-kinds.test.ts (ADR 0042).
11. Phone: measure its bytes on the phone tier; the total must not grow (offset it elsewhere).
12. Tests: a unit test of the node setup and geometry; a scene-census line and an e2e assertion for presence.
13. Visual check: a real-GPU plate (bun run shots) from an oblique angle; snapshots in the PR (pr-snapshots).
14. Docs: docs-upkeep (ledger, the codebook in docs/rendering.md, the guide's layer table in both languages).