coord_for spreads records across C=0,1,2,3 as i grows past 65536,
but the bench assumed everything beyond c=0 landed in c=1. At
scale=200k this asserted (got 65536, expected 134464). Fix: compute
per-bucket expected counts from the coord_for mapping and extend the
total-coverage assertion to include c=2 and c=3.
Verified: fmt clean, clippy -D clean, 100k + 200k both pass.
Co-Authored-By: Hermes Agent (upstage/solar-pro4:free)
