The picked position (lng/lat degrees, height metres above the rendered ground datum).
True when the height is PROVISIONAL — terrain is enabled but its DEM tile hadn't
streamed yet (the query returns ~0 until it does). Callers should re-sample the
lng/lat via map.queryTerrainElevation on the map's next 'idle'.
Where the position came from: a 3D layer surface or the terrain/basemap fallback.
The result of pickScenePosition.