5th Overlays item, generated client-side from the same online Terrarium DEM tiles Terrain shading already fetches (no offline fallback — the plugin's DemSource has no public hook to route through the pmtiles:// protocol the offline basemap uses, so this quietly renders nothing without a network, same as Radar/the Wikipedia layer). A plain vector source/layer under MapLibre's own tile lifecycle, no manual refetch-on-move code needed. Thresholds are the plugin's own generic starting point, not yet tuned against real terrain. Labels skipped for now (font-name matching across the online/offline styles' own glyph sets adds real risk for a nice-to-have, not the actual ask).