feat: publish to crates.io with cargo-binstall-ready cross-platform binaries

[cs50victor]

• Add tag-triggered release workflow: native-runner matrix builds macOS (arm64/x64), Linux (x64/arm64), and Windows (x64) binaries, attaches them to the GitHub Release, then runs cargo publish
• Asset names match cargo-binstall's default probe, so cargo binstall frd fetches prebuilt binaries with no binstall metadata, while cargo install frd still compiles from source
• Replace Unix-only shell-outs (sysctl/df/du) with cross-platform sysinfo plus an iterative target/ tree walk, so non-macOS binaries report real RAM and disk instead of going inert
• Detect Windows executable extensions in have() so cargo-installed tools are found there
• Pin sysinfo to 0.36 deliberately: MSRV 1.75 (free under edition 2024), whereas 0.39 needs Rust 1.95
• Add crate metadata (readme/keywords/categories), strip release binaries, and a README with install/usage docs

Before merge: add a crates.io CARGO_REGISTRY_TOKEN repo secret. Cut a release with git tag v0.1.0 && git push origin v0.1.0.

Validation: fmt + actionlint clean; cargo test; cargo check on the Windows and Linux targets; cargo publish --dry-run packaged clean; RAM/disk output cross-checked against sysctl/df.