Shades the visible map by whether each point is currently blocked from
the sun by surrounding terrain (shademap.app-style), computed fully
offline via the same Terrarium DEM tiles the Terrain shading layer
already renders, plus this session's sun-position math and Radar's
checkbox+expandable UI shape.
dem.js pulls elevation-at-a-point out on its own since TODO.adoc's
slope readout and solar calculator will need the same lookup.
randy
c9561a5489