Pin Cargo dependencies to exact latest versions

[ryanfowler]

Summary

• Pin every direct dependency in Cargo.toml to an exact x.y.z version.
• Refresh Cargo.lock to the latest Rust 1.96-compatible resolved set.
• Keep feature flags and target-specific dependency sections unchanged.

Testing

• cargo fmt
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --all-features