Sites in input order

Sites are sorted for the sweep algorithm. Each site’s index points back to its original input point, so create a reverse lookup when input order matters.

const jcv_site* sites = jcv_diagram_get_sites(&diagram);
const jcv_site** sites_by_input = calloc(
    (size_t)num_points, sizeof(*sites_by_input));

for( int i = 0; i < diagram.numsites; ++i )
    sites_by_input[sites[i].index] = &sites[i];

const jcv_site* site = sites_by_input[input_index];

Allocate the lookup for the original point count. Entries can remain NULL for points pruned as duplicates, outside the bounds, or by the clipper. Site pointers remain valid until jcv_diagram_free; free the lookup separately.