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- New lib/cascade.ts: detect when PATCH extends endTime beyond original, query same-groomer downstream active appointments, and shift them forward by (overrunEnd + buffer − downstreamStart). Propagates through the chain until no more conflicts. - Cascade result (shifted[], flaggedForReview[], cascadeLog[]) included in the PATCH response when a shift occurs. Clients receive reschedule email notification. Out-of-business-hours shifts are flagged rather than auto-applied. - Added cascadeDelay() and cascadeOnStatusOverrun() helpers. - Cascade wired into the this_only PATCH path in appointments.ts. Tests: cascade.test.ts UAT: apps/api/UAT_PLAYBOOK.md §2 Refs: GRO-1175, GRO-1162-G Co-Authored-By: Paperclip <noreply@paperclip.ing>