#96 polish: dir: src H1; relative Atom feed links

What

Two small leftovers from the cosmetic review:

• Directory pages still show a debug-ish H1: dir: src (src/build.rs:1311). It should be just src (or src/ with breadcrumb context).
• feed.atom entry links are relative (<link href="/commit/..."/>). Atom consumers expect absolute IRIs; relative resolution is inconsistent across readers.

Fix

• Render the directory name without the dir: prefix.
• Make feed links absolute when --public-origin is set; relative fallback otherwise. Keep escaping.