Exact 2D dominated hypervolume against a fixed reference point. Sorts
points by the first minimization-oriented objective ascending, then
sweeps and accumulates the dominated rectangle area against the
reference. Points that don't strictly dominate the reference are
ignored. Panics with a clear message if the objective space does not
have exactly two objectives (spec §14.2).
Tests cover a known-area front, the no-coverage case, and the panic on
non-2D problems.
Standard Schott spacing: for each front point compute the Manhattan
distance to its nearest neighbor on minimization-oriented objective
values; the spacing metric is the population standard deviation of
those nearest-neighbor distances.
Returns 0.0 for empty or single-point fronts (spec §14.1).
swaits