feat(bus): host-local UNIX-socket pub/sub worker (DEBT-029)

Land the `decnet bus` worker and `get_bus()` factory. Transport is a
host-local UNIX-domain socket (0660, group=decnet); authz is the file
mode. Wire framing is a tiny verb-line + 4-byte-BE length + orjson body.
NATS-style wildcard topics (`*`, `>`). At-most-once, fire-and-forget —
DB stays the source of truth. `FakeBus` / `NullBus` for tests and the
disabled path. Cross-host federation is deferred to a future
`--bridge-tcp` mode; DEBT-030 is master-only and unblocked.

decnet/bus/__init__.py

@@ -0,0 +1,18 @@
+"""DECNET ServiceBus — pub/sub notification substrate.
+
+The bus is the notification layer for DECNET's worker constellation.  The DB
+remains the source of truth for anything durable; the bus carries "something
+happened, go look" events.  Delivery is at-most-once, fire-and-forget.
+
+Consumers call :func:`get_bus` from :mod:`decnet.bus.factory`; never import
+transport implementations directly.  The factory selects the backend via
+``DECNET_BUS_TYPE`` (``nats`` or ``fake``) and honors ``DECNET_BUS_ENABLED``.
+
+Topic hierarchy is defined in :mod:`decnet.bus.topics` and locked early so
+consumers can subscribe with stable wildcard patterns.
+"""
+from __future__ import annotations
+
+from decnet.bus.base import BaseBus, Event, Subscription
+
+__all__ = ["BaseBus", "Event", "Subscription"]