Move fmt, clippy, tests, cargo package verification and the offline mock
smoke into flake checks built with crane, so the crate dependency build is
its own derivation that is reused until Cargo.lock changes. CI runs a single
nix flake check and restores /nix/store from the GitHub Actions cache via
nix-community/cache-nix-action, replacing the rustup + cargo test job.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013cfWL2GVg97oLfkNvPLaci