One undocumented directive, and an API version to match verified

Two facts and one absence. The directive exists — probing the binary accepted it, while –help and the stock configuration file both omit it, which is how eighteen directives in this catalogue were found. The header declares MQ_API_VERSION 8, so a plugin built against an older MailerQ is a compatibility question rather than a certainty. The absence is our stand: no plugin was built or loaded, so nothing here is verified beyond the directive being accepted. That is deliberate — loading unreviewed C into the engine on a node we were using for measurement would have made every other measurement suspect. For Hub the operational point is narrow and useful: a node with a non-empty plugin-directory is running code we did not audit, in the engine's own process, on the incoming mail path. Reading that directive during a node audit is one line, and it is the difference between a node we can reason about and one we cannot.