v0.2.1 — on_turn per-phase progress callback

Adds the native on_turn(turn_index, max_turns, role) callback for per-phase progress (auditor / target / judge), wired through ModelAuditor.run_scenario, run_async, and AuditExperiment.run_scenario_reps.

This is a release-only bump: the feature (8aa04ce) landed on main after the 0.2.0 PyPI release.