feat(workers): bus-backed Workers panel (registry, control, installed flag)

Ships the backend half of Config → Workers:

* Worker registry aggregates `system.*.health` + `system.bus.health`
  heartbeats into a last-seen dict; OK / STALE / UNKNOWN tiers drop
  out of a 90s window (3× the 30s heartbeat interval).
* `GET /api/v1/workers` returns the snapshot plus `bus_connected`
  (so the UI can explain "all UNKNOWN" when the bus socket is down)
  and a per-row `installed` flag populated from
  `systemctl list-unit-files decnet-*.service` (cached 30s).
* `POST /api/v1/workers/{name}/stop` publishes a stop intent on
  `system.<name>.control`; workers listen via the shared control
  listener in `bus/publish.py`.
* Heartbeat + control listener wired into collector / profiler /
  sniffer / prober / mutator worker loops. API self-heartbeats too
  so the panel always has one ground-truth row.
* Topic helper `system_control(name)` + tests covering builder
  validation, control listener shutdown path, and the API surface
  (auth gating, bus-connected field, unknown-name 404).

Adds `StartFailure` / `StartAllResponse` models in anticipation of
the upcoming start endpoints (DEBT-034).

tests/bus/test_topics.py

@@ -61,3 +61,14 @@ def test_attacker_builder_rejects_empty() -> None:
 def test_system_health_builder() -> None:
     assert topics.system_health("sniffer") == "system.sniffer.health"
     assert topics.system_health("mutator") == "system.mutator.health"
+
+
+def test_system_control_builder() -> None:
+    assert topics.system_control("mutator") == "system.mutator.control"
+    assert topics.system_control("collector") == "system.collector.control"
+
+
+@pytest.mark.parametrize("bad", ["", "has.dot", "has*wildcard", "has>wild", "with space", "with\ttab"])
+def test_system_control_rejects_bad_segments(bad: str) -> None:
+    with pytest.raises(ValueError):
+        topics.system_control(bad)