feat(spmc): add a competing queue

[tisonkun]

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.