[codex] Prepare release metadata and MSRV checks

[Bathtor]

What changed

This follow-up to the dependency update pass prepares the crate for the next release.

It:

• fixes the benchmark RNG integration after the rand dev-dependency update
• adds an explicit feature-selection error for the cancellable wheel when no hasher or multiple hasher features are enabled
• simplifies the cancellable wheel hasher selection through an internal map alias
• polishes crates.io and rustdoc-facing metadata and wording
• removes redundant intra-doc link targets so cargo doc runs cleanly
• adds a dedicated MSRV CI job pinned to Rust 1.94.1

Why

The dependency update exposed a benchmark break due to the rand ecosystem version split, and the crate now declares an MSRV and a stricter feature contract. This PR makes those guarantees explicit in code, documentation, and CI.

Impact

Users get:

• working benchmarks with the updated dev-dependencies
• a clear compile-time error for unsupported hasher feature combinations
• cleaner generated documentation
• explicit MSRV coverage in CI

Validation

I ran:

• cargo test --benches --no-run
• cargo test
• cargo test --no-default-features --features fnv-hash,uuid-extras
• cargo test --no-default-features --features sip-hash,uuid-extras
• cargo check --no-default-features and confirmed it fails with the intended explicit feature error
• cargo doc --no-deps