Page-context metadata extension

House docs schema is attributes first: page-audience, page-usage-context, page-orig-author, page-last-author, optional page-last-edited, plus the expanded catalog in @antora-supplemental/page-context.

Facto registers the extension and sets page-context-active.

Agents keep an ifndef::page-context-active[] body fallback for audience / authors / last updated only (attr refs); do not hard-code the extended catalog into the body.

Hub playbooks set page-context-active alongside the extension.