refactor(prober): ActiveProbe ABC + ActiveProbeMeta registry

Replace _jarm_phase / _hassh_phase / _tcpfp_phase boilerplate (3×~50
lines of identical port-iteration logic) with a metaclass-registered ABC.
Adding a new port-iterating active probe is now one class + three methods.

- decnet/prober/base.py: ActiveProbeMeta auto-registers subclasses by
  probe_name; ActiveProbe ABC enforces run/syslog_fields/publish_payload
  with env-driven DECNET_PROBE_PORTS_<NAME> port override.
- decnet/prober/probes/{jarm,hassh,tcpfp}.py: concrete probe classes.
- decnet/prober/worker.py: single _run_probe driver replaces the three
  phase functions; _probe_cycle iterates ActiveProbeMeta.all(); drops
  the ports=/ssh_ports=/tcpfp_ports= kwargs from prober_worker.
- IPv6 leak and TLS cert capture stay as special cases (different call
  shapes; intentionally outside the registry).
- tests/prober/test_active_probe_registry.py: registry contents, sort
  order, priority-10 override, ABC contract per probe class.
- tests/prober/test_run_probe_driver.py: dedup, success, None-skip,
  exception, rotation, publish paths for _run_probe.
- tests/prober/test_prober_worker.py: updated patch targets and
  _probe_cycle call sites; port control via monkeypatch.setattr.

decnet/prober/probes/__init__.py

@@ -0,0 +1,4 @@
+# Import all probe modules to trigger ActiveProbeMeta registration.
+from decnet.prober.probes.hassh import HasshProbe as HasshProbe
+from decnet.prober.probes.jarm import JarmProbe as JarmProbe
+from decnet.prober.probes.tcpfp import TcpfpProbe as TcpfpProbe