`rust/src/sliding_sync/extensions/relevant.rs` holds the matching of an
extension's `lists` and `rooms` against the response. Phase 5 uses it,
and it is cheap to port now. A list without exactly one `SYNC`
operation fails, as Python's assertion does: that is a list with no
ranges or several, under an extension that names it.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01L51DoxB8VnH8UjXJFDgpNG