ldn: wait for the host's reply when joining a network

Connect slept for a second while holding packet_mutex, so the host's
SyncNetwork reply could not be handled until Connect had already
returned success in StationOpened. Players could see each other's
group and enter it, but never actually met.

Wait on a condition variable instead, accept the reply only from the
selected host and session, and fail after five seconds. Check packet
sizes and senders, stop duplicate joins from taking another station,
and seed session IDs so separate networks don't share one.

The SDL frontend never called Network::Init, so its multiplayer
option could not join a room. Initialize and shut down networking
around the core.

Assisted-by: Sol (implementation)
Assisted-by: Claude Opus 5.5 (code review)

0 files changed, 0 insertions, 0 deletions