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/fake.py

@@ -0,0 +1,183 @@
+"""In-process bus transports.
+
+* :class:`FakeBus` — real pub/sub semantics without touching a socket.  Used
+  by unit tests and anywhere ``DECNET_BUS_TYPE=fake`` is set.  Lets code
+  that depends on the bus be exercised entirely inside a single event loop,
+  matching the DECNET testing convention of not opening real network
+  sockets from unit tests.
+* :class:`NullBus` — no-op.  Returned by :func:`~decnet.bus.factory.get_bus`
+  when ``DECNET_BUS_ENABLED=false`` so workers can start cleanly in dev
+  environments where no bus daemon is running.  Publishes are dropped;
+  subscriptions yield nothing and close cleanly.
+"""
+from __future__ import annotations
+
+import asyncio
+from typing import Any
+
+from decnet.bus.base import (
+    BaseBus,
+    Event,
+    Subscription,
+    _CLOSE_SENTINEL,
+    matches,
+)
+from decnet.logging import get_logger
+
+log = get_logger("bus.fake")
+
+# Per-subscriber bounded queue: backpressure policy is drop-oldest so a slow
+# consumer cannot stall publishers (the invariant — DB is the source of
+# truth — makes dropped events acceptable).
+_DEFAULT_QUEUE_SIZE = 1024
+
+
+# ─── FakeBus ─────────────────────────────────────────────────────────────────
+
+
+class _FakeSubscription(Subscription):
+    """Subscription backed by an :class:`asyncio.Queue` fed from
+    :meth:`FakeBus.publish`.  Unregisters itself on close."""
+
+    def __init__(self, bus: "FakeBus", pattern: str, queue: "asyncio.Queue[Any]") -> None:
+        super().__init__(pattern)
+        self._bus = bus
+        self._queue = queue
+
+    async def __anext__(self) -> Event:
+        if self._closed:
+            raise StopAsyncIteration
+        item = await self._queue.get()
+        if item is _CLOSE_SENTINEL:
+            raise StopAsyncIteration
+        return item
+
+    async def _aclose(self) -> None:
+        self._bus._unregister(self)
+        # Unblock any pending __anext__ waiter.
+        try:
+            self._queue.put_nowait(_CLOSE_SENTINEL)
+        except asyncio.QueueFull:
+            pass
+
+
+class FakeBus(BaseBus):
+    """In-process pub/sub.
+
+    Publishes iterate every active subscription and enqueue the event on
+    the ones whose pattern matches the topic.  If a subscriber's queue is
+    full, the oldest event is discarded to make room — same at-most-once
+    semantics as the real UNIX-socket transport.
+    """
+
+    def __init__(self, queue_size: int = _DEFAULT_QUEUE_SIZE) -> None:
+        self._queue_size = queue_size
+        self._subs: list[_FakeSubscription] = []
+        self._connected = False
+        self._closed = False
+        self._lock = asyncio.Lock()
+
+    async def connect(self) -> None:
+        self._connected = True
+
+    async def publish(
+        self,
+        topic: str,
+        payload: dict[str, Any],
+        *,
+        event_type: str = "",
+    ) -> None:
+        if self._closed:
+            raise RuntimeError("publish on closed bus")
+        event = Event(topic=topic, payload=payload, type=event_type)
+        async with self._lock:
+            targets = [s for s in self._subs if matches(s.pattern, topic)]
+        for sub in targets:
+            _enqueue_drop_oldest(sub._queue, event)
+
+    def subscribe(self, pattern: str) -> Subscription:
+        if self._closed:
+            raise RuntimeError("subscribe on closed bus")
+        queue: asyncio.Queue[Any] = asyncio.Queue(maxsize=self._queue_size)
+        sub = _FakeSubscription(self, pattern, queue)
+        self._subs.append(sub)
+        return sub
+
+    def _unregister(self, sub: _FakeSubscription) -> None:
+        try:
+            self._subs.remove(sub)
+        except ValueError:
+            pass
+
+    async def close(self) -> None:
+        if self._closed:
+            return
+        self._closed = True
+        # Wake every still-open subscription so iterators unblock cleanly.
+        for sub in list(self._subs):
+            try:
+                sub._queue.put_nowait(_CLOSE_SENTINEL)
+            except asyncio.QueueFull:
+                pass
+        self._subs.clear()
+
+
+def _enqueue_drop_oldest(queue: "asyncio.Queue[Any]", event: Event) -> None:
+    """Put *event* on *queue*, dropping the oldest item if the queue is full.
+
+    Factored out so both FakeBus and the real UNIX server share the exact
+    same backpressure policy.
+    """
+    while True:
+        try:
+            queue.put_nowait(event)
+            return
+        except asyncio.QueueFull:
+            try:
+                dropped = queue.get_nowait()
+                log.warning(
+                    "bus.fake: subscriber queue full, dropped %s", getattr(dropped, "topic", "?")
+                )
+            except asyncio.QueueEmpty:
+                return
+
+
+# ─── NullBus ─────────────────────────────────────────────────────────────────
+
+
+class _NullSubscription(Subscription):
+    """A subscription that never yields and closes immediately on iteration."""
+
+    async def __anext__(self) -> Event:
+        raise StopAsyncIteration
+
+    async def _aclose(self) -> None:
+        return
+
+
+class NullBus(BaseBus):
+    """No-op bus used when ``DECNET_BUS_ENABLED=false``.
+
+    Publishes are silently dropped; subscriptions are empty.  Intended for
+    dev environments where no bus daemon is running — the process starts
+    cleanly, code that publishes doesn't need feature flags, and nothing
+    ever blocks on a subscriber.
+    """
+
+    async def connect(self) -> None:
+        return
+
+    async def publish(
+        self,
+        topic: str,
+        payload: dict[str, Any],
+        *,
+        event_type: str = "",
+    ) -> None:
+        return
+
+    def subscribe(self, pattern: str) -> Subscription:
+        return _NullSubscription(pattern)
+
+    async def close(self) -> None:
+        return