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

benches/greet.rs

@@ -0,0 +1,10 @@
+use criterion::{Criterion, criterion_group, criterion_main};
+use rust_template::greet;
+use std::hint::black_box;
+
+fn bench_greet(c: &mut Criterion) {
+    c.bench_function("greet", |b| b.iter(|| greet(black_box("world"))));
+}
+
+criterion_group!(benches, bench_greet);
+criterion_main!(benches);