Add clap CLI, lib crate, unit test, and criterion benchmark

Minimal scaffold before turning this into a Nix flake template.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018ALEXkc7pTro7tF1WcUuKb

Cargo.toml

@@ -0,0 +1,14 @@
+[package]
+name = "rust_template"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+clap = { version = "4.6.6", features = ["derive"] }
+
+[dev-dependencies]
+criterion = { version = "0.8.2", features = ["html_reports"] }
+
+[[bench]]
+name = "greet"
+harness = false