[Fix] sort inconsistent comparator

[dimensionscape]

Array.sort uses std::stable_sort, which requires the comparator to be a strict weak ordering. Haxe code doesn't always give it one. The common "shuffle" idiom is enough:

array.sort((a, b) -> Std.random(3) - 1);

With a comparator like that, libstdc++'s insertion step can walk off the front of the range it is sorting. In release builds that ends in a crash, a corrupted heap or a hang, for both value arrays and object arrays.

This replaces std::stable_sort in both sort paths with a small merge sort (hx::StableSort) that stays in bounds whatever the comparator returns. A bad comparator just gets an unspecified order, as it would in JS.

For valid comparators nothing changes: it is still a stable, in-place sort, so the result is exactly what std::stable_sort produced. It is no slower on random data, needs about 4% fewer comparator calls, and is 4 to 5 times faster on already-sorted or reversed input.

The new testInconsistentComparator hangs on current master and passes with this change. testSizes checks order and stability around the merge sort's insertion runs.