Derived elevation layer fallback

Shared hillshade and color-relief derive pixels from currently visible, same-epoch child DEMs or cropped parent heights. Pending children retain shading/palette coverage; arriving children regenerate and replace it using actual heights.

Existing paint, ordering, pitch and terrain-draping limits remain. No methods or script options are added. Fallback does not count as loaded; derived pixels are not vector query features and source/layer identities remain stable. Cache estimates include parent fallback bytes. Only current display targets are derived, so invisible or stale-epoch caches cannot be drawn accidentally.

The common color-relief fixture uses a step ramp: parent100m produces actual red pixels and child1000m produces blue. A model also checks hillshade target derivation, epoch/nodata and budget behavior. Neighbour DEM sample fusion, reverse child coverage and complete cross-tile fades are separate. Portable fixture is for local reproduction; applications substitute their service and attribution. Device acceptance and formal release remain separate.

Current DEM coverage and query semantics extend cached-child coverage, mixed-region sampling and exact-target takeover. Evidence from this earlier batch remains historical; full LOD blending is still pending.