High-level pub/sub + request-reply over the agent-mesh transport.
This commit lands the application-level message dispatch surface
([Inbox], [BusMessage]). The high-level [Bus] type that
drives it lands in the next commit.
What's already in place from prior commits:
- Topic namespacing ([
Topic]) — every pub/sub name is scoped to the issuing user's fingerprint, so two unrelated users on the same LAN can never collide. - Replay defense ([
replay::NonceCache] + [replay::SequenceTracker]) — duplicate nonces and out-of-order sequence numbers from a known peer are rejected.