Phase comparison utilities

[jc-macdonald]

Problem

After running a multi-phase Study, there's no built-in way to compare results across phases — e.g., did refinement actually improve the Pareto front?

Proposed

• Study.compare_phases() → summary table of per-phase hypervolume, front size, best objective values
• Optional: IGD+ between successive phase fronts to quantify improvement
• Could return a dataframe or a formatted string

Notes

All the primitives exist (hypervolume, igd_plus, extract_front). This is a convenience wrapper.