Rust CLI taste

# Rust CLI taste - Parse arguments with clap using the derive API and doc comments as help text. Confidence: 0.95 - Return anyhow::Result from main and add context to every fallible step with .context(). Confidence: 0.90 - Define library error types with thiserror and keep anyhow in the binary only. Confidence: 0.85 - Split the crate into a thin main.rs and a lib.rs that holds the real logic for testing. Confidence: 0.85 - Write normal output to stdout and logs, progress and errors to stderr. Co...