feed: Atom feed has no <author>, so it fails RFC 4287 validation #59

Why it matters
feed.xml shipped in #53 is advertised from the homepage via <link rel="alternate" type="application/atom+xml">. RFC 4287 §4.1.1 requires an atom:author either on the feed or on every entry. Without it the W3C Feed Validator rejects the feed, and strict readers can drop it or show the entries with no author.

Evidence

• site/lib/exports.mjs:60-72 builds the feed head (id, title, subtitle, links, updated) and each entry (id, title, link, updated, summary). Neither has an <author>.
• The live https://tmhsdigital.github.io/Free-Game-Dev-Assets/feed.xml contains 0 <author> elements. Everything else checked out: 40 entries, unique ids, valid dates, a rel="self" link.

Suggested approach
Add <author><name>${esc(site.title)}</name><uri>${esc(base)}/</uri></author> at feed level, which covers every entry.

Done when

• The feed passes https://validator.w3.org/feed/ with no errors.
• site/lib/lib.test.mjs asserts the feed contains the required elements: id, title, updated, author, and per-entry id, title, updated.