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.
- Add `MpscQueue` for thread-safe task scheduling.
- Add `ToxRunner` to execute Tox instances in dedicated threads.
- Update `Simulation` to coordinate time steps across multiple runners using a synchronization barrier.
- Refactor `FakeMemory` and `FakeClock` to be thread-safe.
- Update `tox_network` helpers and tests to utilize the threaded runner infrastructure.
Introduces a new testing support library 'testing/support' that provides
a clean, modular, and fully deterministic environment for testing
toxcore components.