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.

decnet/bus/publish.py

@@ -0,0 +1,36 @@
+"""Fire-and-forget publish helper shared across every worker.
+
+Lifted out of ``decnet/mutator/engine.py`` once a second caller showed up
+(DEBT-031).  Keeping one implementation means the "never break the worker
+loop" guarantee is audited in exactly one place.
+"""
+from __future__ import annotations
+
+from typing import Any
+
+from decnet.bus.base import BaseBus
+from decnet.logging import get_logger
+
+log = get_logger("bus.publish")
+
+
+async def publish_safely(
+    bus: BaseBus | None,
+    topic: str,
+    payload: dict[str, Any],
+    event_type: str = "",
+) -> None:
+    """Publish on *bus* without ever raising back at the caller.
+
+    The DB row (or equivalent side-effect) has already been committed by
+    the time a worker calls this; the bus is the notification layer, not
+    the source of truth.  A dropped publish is at most a few seconds of
+    UI latency until the next poll tick.  A raised exception here, by
+    contrast, would crash the worker — which is strictly worse.
+    """
+    if bus is None:
+        return
+    try:
+        await bus.publish(topic, payload, event_type=event_type)
+    except Exception as exc:  # noqa: BLE001
+        log.warning("bus publish failed topic=%s: %s", topic, exc)

decnet/bus/topics.py

@@ -11,8 +11,12 @@ Token structure (NATS-style, dot-separated):
     decky.{decky_id}.state
     decky.{decky_id}.traffic
     attacker.observed
+    attacker.scored
+    attacker.session.started
+    attacker.session.ended
     system.log
     system.bus.health
+    system.{worker}.health
 
 Wildcards (per :func:`decnet.bus.base.matches`):
 
@@ -47,9 +51,22 @@ TOPOLOGY_STATUS = "status"
 DECKY_STATE = "state"
 DECKY_TRAFFIC = "traffic"
 
+# Attacker event types (second token under the ``attacker`` root).  First
+# sighting, session boundary transitions, and score-threshold crossings
+# published by correlator + profiler.  Consumers typically subscribe to
+# the wildcard ``attacker.>``.
+ATTACKER_OBSERVED = "observed"
+ATTACKER_SCORED = "scored"
+ATTACKER_SESSION_STARTED = "session.started"
+ATTACKER_SESSION_ENDED = "session.ended"
+
 # System event types.
 SYSTEM_LOG = "log"
 SYSTEM_BUS_HEALTH = "bus.health"
+# Worker-health leaf — built per-worker as ``system.<worker>.health`` via
+# :func:`system_health`.  The leaf constant stays the same across workers;
+# the worker name goes in the middle token.
+SYSTEM_HEALTH = "health"
 
 
 # ─── Builders ────────────────────────────────────────────────────────────────
@@ -89,6 +106,31 @@ def system(event_type: str) -> str:
     return f"{SYSTEM}.{event_type}"
 
 
+def attacker(event_type: str) -> str:
+    """Build ``attacker.<event_type>``.
+
+    *event_type* is typically one of ``ATTACKER_OBSERVED``,
+    ``ATTACKER_SCORED``, ``ATTACKER_SESSION_STARTED``,
+    ``ATTACKER_SESSION_ENDED``.  Dotted leaves (``session.started``) are
+    permitted — same rationale as :func:`system`.
+    """
+    if not event_type:
+        raise ValueError("attacker topic requires a non-empty event_type")
+    return f"{ATTACKER}.{event_type}"
+
+
+def system_health(worker: str) -> str:
+    """Build ``system.<worker>.health``.
+
+    Worker-health heartbeats live as a nested leaf under ``system`` so
+    consumers can subscribe to ``system.*.health`` for every worker at
+    once, or to ``system.mutator.health`` for a single one.  *worker* is
+    validated as a regular segment — no dots, wildcards, or whitespace.
+    """
+    _reject_tokens(worker)
+    return f"{SYSTEM}.{worker}.{SYSTEM_HEALTH}"
+
+
 def _reject_tokens(*parts: str) -> None:
     """Reject topic segments that would break NATS-style tokenization.