feature: add async.channel

[PyXiion]

Parent: #13

Implement the local coroutine channel primitive described in the parent proposal.

Required API:

local ch = async.channel({ capacity = 32 })
ch:send(value)
ch:trySend(value)
ch:recv()
ch:tryRecv()
ch.handler = function(value) end
for value in ch do end
ch:close()
ch:isClosed()

Acceptance criteria:

• Bounded FIFO queue.
• send suspends when full.
• trySend never suspends.
• recv suspends when empty.
• tryRecv never suspends.
• Handler and iterator are alternative consumers.
• Close drains queued values and wakes suspended operations.
• Values are safely snapshotted.
• Tests cover full, empty, closed, and concurrent behavior.