Require explicit seeds for `Simulation` and `SimulatedEnvironment` to
ensure reproducible test results.
Also:
- Drop packets in `FakeUdpSocket` when the receive queue is full to
prevent unbounded memory growth during stress tests.
- Improve synchronization in `Simulation::run_until` by adding a timeout
to the barrier wait, preventing hangs if a runner is unregistered.
Move core networking types and the Network interface to net, and the
standard OS socket implementation to os_network. Update network to use
these new abstractions.
iphydf