refactor(bus): extract publish_safely + extend topics for DEBT-031

Shared publish_safely helper at decnet/bus/publish.py so the nine
workers about to be wired into the bus don't each copy-paste the
"never raise back at the caller" contract. Mutator drops its private
copy and imports the canonical one.

topics.py gains the attacker.* hierarchy (observed, scored,
session.started, session.ended) and a system_health(worker) builder
for per-worker health heartbeats — both prerequisites for the worker
rollout under DEBT-031.

tests/bus/test_publish.py

@@ -0,0 +1,64 @@
+"""Tests for :mod:`decnet.bus.publish`.
+
+The whole point of ``publish_safely`` is that it never raises back at the
+caller.  These tests pin that contract: ``None`` bus is a no-op, a real
+bus publishes, and a raising bus is swallowed + logged.
+"""
+from __future__ import annotations
+
+import logging
+
+import pytest
+
+from decnet.bus.base import BaseBus, Event, Subscription
+from decnet.bus.fake import FakeBus
+from decnet.bus.publish import publish_safely
+
+
+class _ExplodingBus(BaseBus):
+    """Minimal bus whose ``publish`` always raises."""
+
+    async def connect(self) -> None:  # pragma: no cover - trivial
+        return None
+
+    async def publish(self, topic, payload, *, event_type=""):
+        raise RuntimeError("transport exploded")
+
+    def subscribe(self, pattern: str) -> Subscription:  # pragma: no cover
+        raise NotImplementedError
+
+    async def close(self) -> None:  # pragma: no cover - trivial
+        return None
+
+
+@pytest.mark.asyncio
+async def test_publish_safely_none_bus_is_noop() -> None:
+    # Must not raise.  A worker that couldn't connect at startup passes
+    # bus=None and expects every call to silently no-op.
+    await publish_safely(None, "system.log", {"msg": "hi"})
+
+
+@pytest.mark.asyncio
+async def test_publish_safely_delivers_on_live_bus() -> None:
+    bus = FakeBus()
+    await bus.connect()
+    try:
+        sub = bus.subscribe("system.log")
+        async with sub:
+            await publish_safely(bus, "system.log", {"msg": "hi"}, event_type="log")
+            event = await sub.__anext__()
+            assert isinstance(event, Event)
+            assert event.topic == "system.log"
+            assert event.type == "log"
+            assert event.payload == {"msg": "hi"}
+    finally:
+        await bus.close()
+
+
+@pytest.mark.asyncio
+async def test_publish_safely_swallows_transport_errors(caplog: pytest.LogCaptureFixture) -> None:
+    caplog.set_level(logging.WARNING, logger="bus.publish")
+    # The exploding bus would crash the caller without publish_safely.
+    # After wrapping, the caller sees nothing but a log line.
+    await publish_safely(_ExplodingBus(), "system.log", {"msg": "hi"})
+    assert any("bus publish failed" in rec.message for rec in caplog.records)

tests/bus/test_topics.py

@@ -40,3 +40,23 @@ def test_segment_validation(bad: str) -> None:
         topics.topology_status(bad)
     with pytest.raises(ValueError):
         topics.decky(bad, topics.DECKY_STATE)
+    with pytest.raises(ValueError):
+        topics.system_health(bad)
+
+
+def test_attacker_builder() -> None:
+    assert topics.attacker(topics.ATTACKER_OBSERVED) == "attacker.observed"
+    assert topics.attacker(topics.ATTACKER_SCORED) == "attacker.scored"
+    # Dotted leaf is intentional — same as system.bus.health.
+    assert topics.attacker(topics.ATTACKER_SESSION_STARTED) == "attacker.session.started"
+    assert topics.attacker(topics.ATTACKER_SESSION_ENDED) == "attacker.session.ended"
+
+
+def test_attacker_builder_rejects_empty() -> None:
+    with pytest.raises(ValueError):
+        topics.attacker("")
+
+
+def test_system_health_builder() -> None:
+    assert topics.system_health("sniffer") == "system.sniffer.health"
+    assert topics.system_health("mutator") == "system.mutator.health"