New achievements table: stage_completed/curriculum_completed/assessment_completed/
capstone_completed, idempotent (WHERE NOT EXISTS guard, not a unique-index dance — simple and
correct at this write volume). Not a certificate — a durable internal record Phase 4/5 can
render one from later. recordAssessmentAchievement only fires on an actual pass, defensively,
even though today's only completion_rule already guarantees that.
