feat(spmc): add a competing queue

Parent: #206

## Goal

Add bounded and unbounded SPMC queues with one non-cloneable producer and cloneable receivers competing for each accepted value.

## Direction

- Keep the public path at `asyncband::spmc`.
- Preserve a static single-producer contract through a non-cloneable sender and exclusive send methods.
- Reuse the competing-consumer core only when doing so preserves the simpler public capability model and acceptable performance.

## Acceptance criteria

- Cover exact-once delivery, receiver competition, cancellation handoff, disconnection, and bounded backpressure.
- Benchmark the skewed 1P/8C shape that exposes wake-all thundering-herd behavior.
- Avoid sustained order-of-magnitude regressions against comparable competing-consumer channels.

Depends on #208 and should follow the competing-consumer design established in #211.

Start with the asyncband::spmc public entry point, then read the dependency in #208 and the competing-consumer design in #211. Define bounded and unbounded SPMC queues with exact-once competing delivery, cancellation handoff, disconnection, and backpressure. Validate the 1P/8C benchmark and avoid sustained order-of-magnitude regressions.

Written by the indexing model from the issue text.