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.
e7809e1ce2