docs: document reactive and network primitives

[PyXiion]

Parent: #13

Document ref, async.channel, net.shared, and net.channel after the APIs stabilize.

Examples should cover:

• Local producer and consumer channels.
• Bounded channels and trySend.
• Channel handlers and iteration.
• Scalar and table refs.
• Server-wide shared state.
• Player-bound shared state.
• Client readiness and pending handles.
• Two-way player-bound channels.
• Server-side sender context.
• Disconnect behavior.
• Client-writable state warnings.

Add tests for the documented examples where practical.