Adds engine.compute_partner_grouped(period, account_type=...) that returns per-partner aggregations with aging buckets (current/1-30/ 31-60/61-90/90+). SQL-direct for performance — single GROUP BY query with conditional sum per bucket. Foundation for the 3 partner-grouped reports landing in commit 3: Aged Receivable, Aged Payable, Partner Ledger. Made-with: Cursor