e7809e1ce2
Detect content/_404.md as special page (underscore prefix convention), store in SiteManifest.page_404, render to 404.html in output root. Add collect_tags() grouping all tagged content into BTreeMap for deterministic output. render_tag_page() renders through tags/default.html template. Add 5 tests. Test suite: 78 → 83, all passing.