Claim Receipt

Claim the next eligible backlog row and return a signed claim receipt.

The dependency gate is enforced by :class:ClaimFilter: a row is offered only when its depends_on are all in completed_ids. The granted claim is mirrored into the audit chain via the existing record_task_claim_receipt (no new event type), and the returned receipt embeds that event's chain head so the claim verifies offline. A filter matching no eligible row returns a signed refusal receipt.