refactor(quickadapter): consolidate custom distance metrics in Pareto front selection

refactor(quickadapter): consolidate custom distance metrics in Pareto front selection

Extract shared distance metric logic from _compromise_programming_scores and
_topsis_scores into reusable static methods:

- Add _POWER_MEAN_MAP class constant as single source of truth for power values
- Add _power_mean_metrics_set() cached method for metric name lookup
- Add _hellinger_distance() for Hellinger/Shellinger computation
- Add _power_mean_distance() for generalized mean computation with validation
- Add _weighted_sum_distance() for weighted sum computation

Harmonize with existing validation API using ValidationMode and proper contexts.