Parent terrain tile fallback

Shared raster-dem terrain uses the nearest available ancestor from the same resource epoch while target child tiles are pending. Cropping and height resampling retain a real surface over the current view. Existing setTerrain needs no additional option.

Fallback RGBA and height data cost8 bytes per pixel within the map-wide64MiB tile budget. Heights use bilinear sampling and preserve nodata. Updated URL/config epochs never reuse old tiles. Without a suitable parent, data remains unavailable. Arriving children replace fallback geometry without duplicate drawing.

getSourceInfo.tileState.fallbackTiles counts fallback targets; readyTiles counts actual children and fallback never makes loaded true. queryTerrainElevation uses the highest cached original DEM. Display project/unproject uses the current cropped fallback grid, so sample precision can differ. Existing valid skirts and surface-only picking still apply.

This stage covers real terrain surfaces. Derived hillshade/color-relief fallback is connected within dem-color-fallback; reverse child coverage, cross-tile fades, DEM sample stitching and adaptive LOD remain separate. Portable script uses an owned local generated DEM: parent100m and child1000m. Applications must substitute their own service and show attribution. Native and both browser backends record actual wait/pixels/heights/replacement; 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.