feature: add net.channel

[PyXiion]

Parent: #13

Add the network-backed channel implementing the async.channel contract.

Required API:

local ch = net.channel("events", {
  player = player,
  capacity = 32,
})

ch:send(value)
ch:trySend(value)
ch.handler = function(value, sender) end
for value, sender in ch do end

Acceptance criteria:

• Reliable ordered delivery by default.
• Bounded per-recipient queues.
• send suspends when full.
• trySend never suspends.
• Server receives include the sending player.
• Player binding routes only between server and the bound player.
• Server-wide sends use independent per-player queues.
• Handler and iterator are alternative consumers.
• Disconnect closes the channel cleanly.
• Serialization failures reject sends.