test(channel): establish ecosystem performance baselines #208

Parent: #206

Goal

Define repeatable performance baselines for channel work against semantically similar Rust implementations.

Scope

Cover ready paths and concurrent bounded and unbounded workloads for these shapes:

• one producer and one consumer;
• many producers and one consumer;
• one producer and many competing consumers;
• balanced many-producer, many-consumer contention;
• broadcast fanout and producer contention when broadcast exists;
• latest-state notification when watch exists.

Use comparable semantics and document exceptions such as Tokio broadcast's overwrite and lag behavior. Measure multiple participant counts so wake-all or contention cliffs are visible.

Acceptance criteria

• The benchmark methodology and peer versions are documented.
• Results are reproducible locally through the repository workflow or a documented companion workflow.
• Sustained regressions above 3x are investigated, and order-of-magnitude regressions block an implementation from being accepted unless the semantic tradeoff is explicit.
• Third-party benchmark dependencies do not become runtime dependencies of asyncband.