Answer a pending interaction
Answer a pending interaction through the authenticated human door.
Audience gate (after the existence/format/status guards): a question’s
audience identity OR any unrestricted caller (the operator can always unblock
a stuck question) may answer; every OTHER restricted caller is a loud 403.
An unaddressed question is answerable by any unrestricted caller and by no
restricted caller. This gate is sound ONLY because a restricted caller can never
obtain a question’s callback ticket — the ticket is delivered exclusively over the
configured channel (never on any read/stream frame), so the unauthenticated
callback door stays the sole ticket-bearing surface and no filtered stream leaks
it.

