export connectivity metrics

The visualisation is about the largest connected component growing, but no connectivity metric is
exported. We replay the construction with a union-find over OSM node ids, fed from the `edge_list`
repr strings, to produce `lcc_share`, `lcc_gain` and per-edge `lcc_step_`.

**Ask:** export per-step `lcc_share` from the run that already computes it, and ideally the per-edge
step at which each existing edge joins the largest component.