Fix IndexTreeList.add(index) to allow index == size

[arimu1]

Fixes #1059

Problem

IndexTreeList.add(index, element) called checkIndex(index), which rejects index >= size. That matches access operations (get, set, removeAt) but not insertion: List.add(int, E) allows index == size() for append-style inserts. On an empty list, add(0, e) (and JDK 21 addFirst) therefore threw IndexOutOfBoundsException.

Fix

Use insertion bounds in add(index, element) only: index < 0 || index > size. Leave checkIndex unchanged for read/update/remove paths.

Testing

mvn test -Dtest=IndexTreeListTest#addAtIndexEmptyList,IndexTreeListTest#addAtIndexAppend -Dmdbtest=1

Result: 2 tests passed (darwin, Corretto 8 / Maven default JDK).