Hreflang checker

The language-code check is BCP 47 syntax only. It is never checked against the IANA registry, so a well-formed but unregistered code passes with a note, and grandfathered irregular tags fail the regular grammar. Reachability means the proxy got an answer under HTTP 400; it does not render the page or follow redirects. Reciprocity is URL-based: the target must list a link tag pointing back at the audited address. A truncated response leaves a link-back unconfirmed rather than calling it missing, and nothing past the readable window of a page or target can be seen at all.