Legacy-cube association fix: human-seat/default-worker role selection is
flag-only via one shared predicate, so imported legacy cubes (worker-class
human seat + separate queen role) can be adopted/associated (#218).
Visitor-first README rewrite with verbatim relocations to operator/protocol
references (#217).

Reviewed release candidate a283a2119e85148c95658f292bdca87157153fff
merged as f8a1c3f3907171fc27644b873a17ba234a921616.