ARR/MRR rollforward with churn, expansion, and cohort view

Build an ARR/MRR rollforward for {period}.

Inputs:
- Subscription snapshot data: {subscriptions}
- Churn and expansion events: {customer_events}
- Pricing changes and discounts: {pricing_changes}
- Definitions (ARR vs MRR, gross vs net): {definitions}

Output:
1) Rollforward table: Begin, New, Expansion, Contraction, Churn, End.
2) Gross and net retention metrics with formulas.
3) Cohort retention table (by start month/quarter) if data supports.
4) Data quality checks (duplicates, proration, currency).

Be explicit about definitions used.