[codex] modernize rust dependencies and ci cache

[GTrunSec]

Summary

• update Rust dependencies to current crates.io releases and migrate parser code to nom 8 APIs
• add tracked bench fixtures so cargo test --workspace --all-targets --all-features covers benches cleanly
• tighten GitHub Actions for PR/v0.10 with strict fmt/test/clippy gates and no-nix Rust caching via sccache + rust-cache
• include devenv/pre-commit setup for local reproducibility

Validation

• direnv allow .
• direnv exec . cargo fmt --all -- --check
• direnv exec . cargo test --workspace --all-targets --all-features
• direnv exec . cargo clippy --workspace --all-targets --all-features -- -D warnings
• git diff --cached --check

Notes

• wasm-pack build was not run locally because wasm-pack is not installed in this shell; CI still installs it for the wasm job.