The API reference loads reliably

The API reference could show a missing-specification message depending on which directory the application was started from, and a specification the server could not read was reported the same way as one that was not there.

• The reference finds its specification in every supported layout
• A specification the server cannot read now reports a server error instead of a missing page, so the real fault is visible
• Pointing a deployment at a specific specification file is now authoritative: if that file is absent the reference says so, rather than quietly serving a different contract
• The published specification is checked by a real validator before any change can merge