Support read-only settlement observation for legacy host receipts #78

A Pylon host can retain a capability-proved acquired ownership receipt after process restart without a recovery handle or durable prior socket path. Existing replacement-client cleanup is socket-scoped; host-side socket/PID absence must not substitute for exact worker settlement.

Add a read-only public SDK observation, gated by frozen feature metadata, that uses Prime's descriptor-last cleanup invariant across the exact canonical agent home's socket namespaces. It must never adopt historical ownership, launch work, complete another client's session, signal processes, or expose private identifiers in results. Missing, malformed, changing, nonprivate, or aliased state must fail closed; a registered root must remain blocked even when its process/socket is gone. Recoverable adoption remains separate.

Validate with focused registry cases and a real isolated owned-worker lifecycle. Pylon delivery is tracked in pylon-code/pylon#557.