Diagnose enterprise referral collapses: 48828, 86691, 85067, 90934

Why this matters: Q1 2026 per-merchant decline is heavily concentrated in a few high-volume accounts; account-level root causes likely drive most of the aggregate signal.\n\nScope: For each account, identify whether decline came from cancellation, campaign state/config changes, attribution-path breakage, or merchant business changes; propose recovery action by owner (PM/CS/Eng).\n\nSource signal: projects/referral-team/analysis/2026-04-08-merchant-base-shrinkage-diagnosis.md (Finding 6), projects/referral-team/analysis/2026-04-08-enterprise-account-collapses.md, td-319348 decision brief.