Function syncHillshadeOverlay

Brings the hillshade overlay in line with the basemap that just became active: drawn where that basemap wants it, or gone when it declares none. Called once the basemap is in place, on every activation path.

• An image basemap (tiles, WMS, WMTS, georeferenced image): right above the basemap's layer, under everything else. When that layer is not there — a WMTS still resolving, an apply that failed — there is nothing to lay the shading over, and the overlay is removed.
• A vector basemap: before hillshade.beforeLayer, else before the first symbol layer of its style, else right above the style's last layer.

The overlay already on the map is left untouched when it is the one asked for — its source spec, its paint and the layer it was asked to sit before, kept as a signature in the layer's own metadata. No state lives outside the map: a style change takes the signature away with the layer.

It is NOT declared to the style transform: carried layers are appended above every layer of the incoming style, labels included. A style change drops the overlay, and this puts it back.

With hillshade.onlyWithTerrain the overlay is added hidden or shown, as the map holds a terrain or not at that moment, and follows the terrain from then on. The basemap's terrain is settled AFTER this runs, on every path: the overlay learns of it from the engine.

Never throws: an overlay that cannot be drawn must not keep the basemap switch from finishing.