Link Graph Audit

Build the internal link graph from a set of supplied pages and report its structure. Names pages with no inbound link, pages unreachable by following links from the declared entry points, internal links pointing outside the supplied set, and links that do not resolve at all. Fragments are stripped, since they identify a position rather than a page. Nothing is fetched, so a link to a page that exists and was not supplied is reported as not in the set rather than as broken.