This registers a credential against whatever origin serves this page
and prints a fixture file for t/fixtures/. It is a
development tool, not part of the distribution's runtime: WebAuthn
requires a secure context, so serve it over https or from
localhost.
Every fixture wants a row in t/fixtures/README naming
the authenticator it came from - "iPhone, platform", "YubiKey 5,
roaming" - because a capture whose provenance nobody recorded is a
vector nobody can re-derive.
(nothing captured yet)