In-memory message bus (single-process / test).

Holds an atom vector of registered delivery-fns. `publish` invokes each
synchronously and sums their returned counts. This is the default bus for
single-node deployments and the vehicle for the 2-node cross-instance test
(two services sharing one bus instance via the :bus option).

Redis-backed IMessageBus: routing envelopes travel as Nippy-frozen bytes over
a binary Redis pub/sub channel. publish borrows a pooled connection; the
subscriber owns ONE dedicated connection (not from the publish pool, so a
blocking SUBSCRIBE never starves publishers) on a daemon thread.

Concurrency:
- Singleton subscriber: start-subscriber! is idempotent (running? guard) so a
  node never holds two channel subscriptions (which would double-deliver).
- Reconnect: the daemon loops with backoff, acquiring the connection INSIDE
  the loop so a Redis outage at startup retries instead of killing the daemon
  and leaving the node permanently deaf. Gap messages are missed
  (at-most-once, accepted).

Trust boundary: onMessage thaws Nippy bytes off the channel. Envelopes are
produced only by this framework's own publishers on a trusted Redis instance.
Do NOT point this at a Redis shared with untrusted writers — Nippy thaw of
attacker-controlled bytes is a deserialization risk.