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