Fold peer claim receipts into this node's signed journal (#2558).

The leaderless counterpart to POST /cluster/steal: no node decides who gets what here. Each receipt is folded only after its Ed25519 signature and its chain link both verify, so an unverifiable receipt is never written.

A receipt that does not extend the local head is not merged. It produces a signed fork receipt carrying the divergence entry index, which the response surfaces through forked. Silent merge would be the one failure mode a leaderless design cannot recover from: two partitions would each hold a coherent-looking journal describing incompatible work.

Authorisation reuses the node-heartbeat scope: gossip is a peer-to-peer fleet-membership operation, not an administrative one.