Rebuilt ripgrep's directory walker to understand how it actually works

Did the reverse-engineer lesson on ripgrep and got a bit obsessed with how it walks directories so fast. Stripped my version down to about 300 lines with a comment on basically every decision.

The .gitignore handling was the bit that clicked for me. It doesn't just skip files at the end, it prunes whole folders early so it never even opens them.

Best way I've found to actually learn Rust instead of doing tutorial exercises.