feat(mpmc): add a competing queue

[tisonkun]

Parent: #206

Goal

Add bounded and unbounded MPMC queues in which cloneable receivers compete for each value and every accepted value is delivered exactly once.

Direction

• Keep the public path at asyncband::mpmc.
• Use targeted waiter notification with cancellation handoff rather than waking every sender or receiver.
• Keep bounded capacity strict and unbounded sends synchronous.
• Keep implementation policy and storage types private.

Acceptance criteria

• Cover clone counts, ordering, disconnection, draining, cancellation, exact-once delivery, and bounded progress under contention.
• Benchmark 1P/1C, 1P/8C, 8P/1C, and 8P/8C for bounded and unbounded modes.
• Avoid sustained order-of-magnitude regressions against semantically comparable MPMC channels.

Depends on #208. The implementation may share narrowly scoped machinery established by #209 without forcing single- and competing-consumer paths into one generic policy framework.