The objective-comparison branch of `pareto_compare` materialized two
`Vec<f64>`s per call via `ObjectiveSpace::as_minimization`. Because
`pareto_compare` runs O(n²) times across the multi-objective algorithms,
that per-call allocation pair dominated the whole `compare` workload.
Replace it with an allocation-free per-objective scan that branches on
`Objective::direction` directly: for a Maximize axis "a beats b" is just
`av > bv`, bit-identical to `-av < -bv` after orientation. The result is
unchanged for every input.
Whole-program callgrind Ir for the `compare_profile` benchmark:
357,060,633,544 -> 221,836,742,708 (-37.87%).
swaits
740965958f