refactor(broadcast): remove overflow channel

[tisonkun]

Summary

• remove the lossy broadcast::overflow channel and its broadcast Cargo feature
• remove its dedicated tests, benchmarks, API documentation, and feature wiring
• remove the private poisoned-lock wrapper that was used only by the overflow buffer

Rationale

broadcast::overflow implements the sliding/drop-oldest behavior deferred in #167. Its observable capacity, lag, resume-position, and eviction semantics need a clearer contract, and there is no concrete downstream requirement that justifies carrying the existing API into 0.7.

Removing it now leaves room for the lossless bounded and unbounded broadcast APIs described in #167 without preserving an incompatible public path or error model. Shared waiter infrastructure remains in place because other primitives still use it.

Validation

• cargo run -p x -- build --locked
• cargo run -p x -- test
• cargo run -p x -- check
• cargo run -p x -- lint
• cargo run -p x -- semver --release-version 0.7.0