chore(bus): add scripts/bus/ smoke + manual test helpers

start.sh boots a local bus on /tmp (no root, no decnet group).
sub.py / pub.py are thin CLIs over UnixSocketBus for manual poking.
smoke.sh is a self-contained end-to-end check — spawns a worker,
subscribes, publishes, asserts delivery, cleans up.

scripts/bus/start.sh

@@ -0,0 +1,11 @@
+#!/usr/bin/env bash
+# Start a local `decnet bus` worker for manual smoke-testing.
+# Uses /tmp so it works without root and without the `decnet` POSIX group.
+# Usage: scripts/bus/start.sh [heartbeat-seconds]
+set -euo pipefail
+
+SOCK="${DECNET_BUS_SOCKET:-/tmp/decnet-bus.sock}"
+HEARTBEAT="${1:-3}"
+
+echo "bus: socket=${SOCK} heartbeat=${HEARTBEAT}s  (Ctrl-C to stop)"
+exec decnet bus --socket "${SOCK}" --group "" --heartbeat "${HEARTBEAT}"