#89 file paths in hrefs are not percent-encoded (# ? % break navigation)

What

href_esc (src/build.rs:407) is only HTML escaping; file path segments are never percent-encoded. Git allows #, ?, %, spaces and quotes in filenames.

• a#b.txt → /files/a#b.txt.html requests /files/a → 404
• q?x.txt → /files/q?x.txt.html requests /files/q → 404
• spaces happen to work because browsers encode them; % can be rejected as a bad escape by serve
• raw links, breadcrumb links and directory rows share the helper

Reproduced

A fixture repo with a#b.txt, a b.txt, q?x.txt; navigation 404s for #/? while /files/a%23b.txt.html returns 200.

Impact

A repo-controlled filename can make a file unreachable from the UI (or hide it from review); not XSS because esc still runs.

Fix

Add a path-segment percent-encoder ([A-Za-z0-9-._~] plus /; everything else %XX UTF-8) and use it for hrefs only. Keep displayed text escaped. Test #, ?, %, space, non-ASCII, quote.