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

src/lib.rs

@@ -0,0 +1,13 @@
+pub fn greet(name: &str) -> String {
+    format!("Hello, {name}!")
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn greets_by_name() {
+        assert_eq!(greet("world"), "Hello, world!");
+    }
+}

src/main.rs

@@ -0,0 +1,15 @@
+use clap::Parser;
+use rust_template::greet;
+
+#[derive(Parser)]
+#[command(version, about)]
+struct Cli {
+    /// Name to greet
+    #[arg(default_value = "world")]
+    name: String,
+}
+
+fn main() {
+    let cli = Cli::parse();
+    println!("{}", greet(&cli.name));
+}