CDN-363: The Challenge Rollback Ledger

CDN-363 defines the public rollback path for GCFT replication updates. It shows that claim states can only be suspended, retained, or rolled back through accepted counter-receipts that target a prior mutating update event, include the required counter-artifact bundle, replay the locked threshold rule, and enter an append-only challenge hash chain. Rejected or unanchored challenges remain visible as no-op audit events without mutating the claim register.