Add link-phase validation to @page-tags
Page_tags now uses register_with_link so it gets access to the
cross-reference environment at link time. For each tag, it calls
Env.lookup_page_by_path with (TCurrentPackage, ["tags"; tag]) to
verify that a corresponding tag page exists under site/tags/.
If the lookup fails, the build crashes with a clear message:
@page-tags: no page found for tag 'foo'. Create
site/tags/foo.mld before using this tag.
This enforces the invariant that tag chip links always resolve —
you can't use a tag until you've created its index page.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>