CultMeshRudpSocketOptions.BindHost defaulted to 0.0.0.0 for servers and
clients alike (a58daa54, so clients could reach remote routes). That bound an
IPv4 socket for every client, so a client for an IPv6 endpoint could not send,
and a client for a loopback endpoint listened on every interface.
BindHost is now nullable and unset by default. Unset, a client binds loopback
for a loopback endpoint and the unspecified address of the endpoint's family
(0.0.0.0 or ::) otherwise, matching cultnet-rs, cultnet-ts, cultnet-py and
cultmesh-kotlin. A server keeps binding 0.0.0.0. An explicit BindHost wins for
both. The test that pinned the old default's value is replaced by tests of
what each endpoint binds.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>