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_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"