feat(channel): implement channel taxonomy

[tisonkun]

Closes #167. Supersedes #117.

Summary

• Keep direct public paths such as asyncband::mpsc, asyncband::spmc, and asyncband::broadcast, while collecting their implementations in a private channel source tree.
• Preserve the existing oneshot and MPSC APIs and algorithms; add bounded and unbounded SPSC, SPMC, and MPMC competing queues, lossless SPMC and MPMC broadcast channels, and a latest-state watch channel.
• Encode topology in nominal endpoint types: only the endpoints that support multiple producers or competing consumers are cloneable.
• Keep bounded and unbounded semantics explicit. Bounded channels apply backpressure without dropping accepted values; sliding/drop-oldest and lag semantics remain deferred.

Design Notes

• This supersedes the earlier unbounded broadcast work in feat(broadcast): add unbounded MPMC channel #117. Its implementation is not carried over, while Orthur's contribution is retained through the feature commit's Co-authored-by trailer.
• MPSC retains the current AtomicWaker receiver path from main, and SPSC reuses that backend. Multi-consumer queues and broadcasts use WaitSet because they must track multiple pending receivers.
• Broadcast is a lossless retained log gated by the slowest live subscription. Its dense cursor registry makes reclamation proportional to current fanout rather than historical subscription high-water.
• User value drops, clones, and wakeups occur outside internal locks. Disruptor/ring and power-of-two constraints stay internal and out of the public API; no current channel semantics require them.
• Latest representative medians from cargo x bench: MPSC unbounded 9.79 ns, MPMC queue unbounded 8.81 ns, SPSC queue unbounded 9.78 ns, broadcast unbounded with one subscription 28.58 ns, and watch send-and-borrow 15.56 ns. Broadcast reclamation with 256 historical subscriptions and one live subscription was 32.81 ns.
• Validated with cargo x lint, cargo x test, cargo x check, cargo x bench, and cargo +1.86.0 check -p asyncband --all-features --all-targets.

[tisonkun]

Closing this bundled implementation in favor of an incremental rollout.

The channel work is now tracked by #206 through independently reviewable sub-issues. The replacement starts with #207, which only moves the existing oneshot and MPSC implementations into the terminal private source layout without changing algorithms or adding channel families.

[tisonkun]

The focused replacement for the first step is now #217. It only establishes the private source layout for the existing oneshot and MPSC implementations and closes #207.