New tables: assessment_definitions (reusable template — pool_spec, duration, hint_policy,
completion/unlock rules, a plain incrementing version bumped only on semantic edits),
assessment_cohort_assignments, assessment_sessions (one per student attempt, full snapshot of
what was drawn and under what rules — never affected by a later definition edit),
assessment_session_scenarios (the drawn set, ordered, with a real FK to scenario_attempts).
Deliberately does not touch/reuse exam_sessions (migration 006) — that was already scoped in
Phase 2B as historical-import-only, never the live mechanism; these tables are the real thing.
drawScenarios: every 'required' scenario id included, plus N random picks per pool group,
shuffled together (tiny seeded PRNG, mulberry32 — no new dependency; the seed is recorded for
audit, never shown to the student before the draw). One assessment at a time per student.
Timer is server-authoritative (expires_at fixed at draw time) and lazily expired — checked
whenever a session is touched, no background cron. Ships two seeded defaults: rootnode-capstone
(Real Ops scenarios required, weighted toward operational ability) and rootnode-quick-assessment
(a general example exam matching the spec's category-draw example).
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