I3sVertexMapper

Maps a raw I3S vertex (Δlon°, Δlat°, Δheight relative to the node origin) to a local Cartesian offset from the tile origin, written into out[offset..offset+2]. Backed by the real projection so tiles share a consistent curved surface (no tangent-plane seams). Δheight arrives in the layer's declared height unit; applying heightUnitToMeters is the mapper's job.