refactor(prober): generalise ActiveProbe registry to absorb Ipv6LeakProbe

ActiveProbe.run/syslog_fields/publish_payload now accept port=None so
non-port-iterating probes can live in the registry. Ipv6LeakProbe replaces
the hand-rolled _ipv6_leak_phase special case in worker.py; it runs last
via priority=999. _probe_cycle no longer has an ad-hoc phase call.

Fixes three stale test files (test_prober_bus, test_prober_rotation,
test_prober_worker) that were broken since the 916b21b6 registry refactor.

decnet/prober/probes/hassh.py

@@ -6,22 +6,24 @@ from decnet.prober.base import ActiveProbe
 from decnet.prober.hassh import hassh_server
 from decnet.telemetry import traced as _traced
 
-DEFAULT_PORTS: list[int] = [22, 2222, 22222, 2022]
+DEFAULT_PORTS: list[int | None] = [22, 2222, 22222, 2022]
 
 
 class HasshProbe(ActiveProbe):
     probe_name = "hassh"
-    default_ports = DEFAULT_PORTS
+    default_ports: list[int | None] = DEFAULT_PORTS
     event_type = "hassh_fingerprint"
     rotation_type = "hassh"
     rotation_hash_key = "hassh_server"
     priority = 100
 
     @_traced("prober.hassh_probe")
-    def run(self, ip: str, port: int, timeout: float) -> dict[str, Any] | None:
+    def run(self, ip: str, port: int | None, timeout: float) -> dict[str, Any] | None:
+        if port is None:
+            return None
         return hassh_server(ip, port, timeout=timeout)
 
-    def syslog_fields(self, ip: str, port: int, result: dict[str, Any]) -> tuple[dict[str, Any], str]:
+    def syslog_fields(self, ip: str, port: int | None, result: dict[str, Any]) -> tuple[dict[str, Any], str]:
         fields = {
             "hassh_server_hash": result["hassh_server"],
             "ssh_banner": result["banner"],
@@ -32,7 +34,7 @@ class HasshProbe(ActiveProbe):
         }
         return fields, f"HASSH {ip}:{port} = {result['hassh_server']}"
 
-    def publish_payload(self, ip: str, port: int, result: dict[str, Any]) -> dict[str, Any]:
+    def publish_payload(self, ip: str, port: int | None, result: dict[str, Any]) -> dict[str, Any]:
         return {
             "attacker_ip": ip,
             "port": port,