Function inject_room_id 

fn inject_room_id(
    map: &Arc<Map>,
    key: &[u8],
    value: &[u8],
    resolve: impl FnOnce(&CanonicalJsonObject) -> Result<OwnedRoomId>,
) -> Result<bool>

Injects room_id into one PDU value that lacks it, sourcing the room from resolve. Returns whether the value was rewritten; false means it already carried a room_id. A cheap HasRoomId probe short-circuits that common case, so only the rewritten PDUs pay the full parse and re-serialize.