insertNid method
Inserts nid at visible position index, shifting the suffix right.
nid must be live.
Implementation
void insertNid(int index, int nid) {
_ensureOrderCapacity(_len + 1);
// Int32List.setRange uses memmove semantics — overlap-safe even when
// source and destination are the same buffer.
_orderNids.setRange(index + 1, _len + 1, _orderNids, index);
_orderNids[index] = nid;
_len++;
_onNidAdded?.call(nid);
_onMutated();
}