Punk::Passkey - capture a fixture

# Capture a passkey fixture

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.

rpId

    (nothing captured yet)