test_monologue_not_duplicated_after_consumption was failing because
poll_snapshot() in test mode returns _test_snapshot() without
consuming _last_snapshot. The carry-forward logic then incorrectly
preserved the monologue. Fix: directly clear _last_snapshot to
simulate the live-mode consumption path.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>