refactor(mpsc): replace the legacy queue backend

[mxsm]

Summary

• Replace the bounded and unbounded std::sync::mpsc backends with MPSC-owned queue implementations while preserving the public sender, receiver, and error contracts.
• Use receiver-local batching for the unbounded queue and a preallocated, generation-stamped slot ring for the bounded queue.
• Keep backpressure, cancellation safety, FIFO ordering, disconnection, and buffered-message draining behavior covered by tests.
• Remove the receiver types' manual Sync implementations and document the bounded slot's localized unsafe invariant.
• Update the changelog for the backend replacement.

Closes #209.

Design Notes

The unbounded queue keeps sender-visible storage and receiver liveness behind one mutex, while the single receiver moves messages into a local batch to reduce shared-lock contention without introducing unsafe code.

The bounded queue allocates its capacity up front and uses per-slot generation stamps. Producers reserve slots through the tail cursor, while the single consumer advances the head cursor; cache padding prevents producer and consumer cursor updates from sharing a cache line.

Unsafe code is confined to initialized slot access and Slot<T>: Sync. Each unsafe operation is paired with a documented reservation, publication, acquisition, and reuse invariant, and the queue is exercised under Miri with multiple producers.

Validation

• cargo x test
• cargo x check
• cargo x miri
• cargo x lint
• cargo x bench

Benchmark Comparison

The following bounded MPSC medians were measured in adjacent runs on the same machine with 16,384 messages per sample.

Producers	Legacy backend	New backend	Change
1	2.235 ms	1.567 ms	-29.9%
2	1.601 ms	1.541 ms	-3.7%
4	1.888 ms	2.095 ms	+11.0%
8	2.235 ms	2.292 ms	+2.6%

The existing ecosystem benchmark continues to cover bounded and unbounded channels with 1, 2, 4, and 8 producers. The full benchmark suite completes without an order-of-magnitude regression or a material bounded-contention regression.