The MSC4133 io.lotus.avatar_decoration value was interpolated into the
CDN URL verbatim, letting a room member steer the path/query of a request
every viewer's browser makes. Accept only slugs present in the catalog;
anything else is treated as no decoration. Unit-tested.
Fixes#64
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA