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.
_route_info() calls _ip_route_get once and returns (on_link, iface);
worker._ipv6_leak_phase now calls it instead of the two separate helpers.
Bare except clauses at _ip_route_get and response parse now log at debug.
Add ipv6_leak.py with solicit_ipv6_leak() — sends ICMPv6 Echo to
ff02::1 on the attacker's iface and returns fe80:: evidence when a
link-local response arrives. Gated on _is_on_link(): skips when
attacker is behind a router (no L2 adjacency).
Add _ipv6_leak_phase() to worker.py (Phase 4 in _probe_cycle).
Phase runs once per attacker IP per cycle (sentinel at port 0 in
ip_probed["ipv6_leak"]) and publishes kind="ipv6_leak" via publish_fn.
Add list_v6_addrs(iface) to network.py: returns [(addr, scope)] for
all IPv6 addresses on an interface, required for source-routing ICMPv6
from the correct link-local address.
