Tracking Issue to deliver the channel family incrementally

[tisonkun]

Goal

Deliver the channel taxonomy agreed in #167 through small, independently reviewable pull requests instead of one bundled implementation.

Working agreements

• Keep public paths at the crate root, such as asyncband::mpsc, while collecting implementations under one private channel source module.
• Preserve existing public behavior unless a subtask explicitly proposes an API change.
• Add one channel family or one implementation concern at a time.
• Compare contention and topology-specific performance with semantically similar Rust channels. Being the fastest is not required, but sustained order-of-magnitude regressions must be treated as implementation defects.
• Keep bounded backpressure and unbounded growth as the initial policies. Sliding, drop-oldest, and lag semantics remain deferred.
• Do not expose queue, ring, Disruptor, or sequencer machinery through the public API.

Delivery

The linked sub-issues define the review sequence. The first step only establishes the terminal private source layout for the existing oneshot and MPSC implementations. New algorithms and channel families follow separately.

Design baseline: #167. Prior broadcast work: #117 and #146.