The bestCandidate function allowed game count to override pairing recency
when both candidates had non-zero LastPairedAt values. Per §6.1, pairing
recency must be the primary criterion with game count only breaking ties.
Also adds 10 tests covering: never-paired preference, oldest-pairing
preference, game count tiebreaking, Pareto distribution verification,
multi-opponent selection, and regression test for the priority inversion.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
