Selection and tracking

Selection { atoms, bonds } is a set of atom indices and normalised bond pairs. add_atom, remove_atom, toggle_atom, add_atoms, add_bond, contains, union, intersect and clear are the editing API.

TrackingTable assigns a stable AtomId to a site and keeps id-to-site and site-to-id lookups. remap rewrites the current indices after an edit and drops ids whose site disappeared, so a selection or measurement can follow an atom across commands.