stablesort: move parent range computation into its own method Now that we have running code, we'll need to avoid too deep recursion. We extract the piece dealing with the parent range before we write more code to warm it.

stablerange: drop the basic inheritance from the 'mergepoint' version All necessary bits are implemented now (except for warmup which we explicitly bypass for now).

stablerange: cache known subrange at the stablerangecached level That way, the 'stablerange_mergepoint' class also benefit from it.

stablerange: use sort cache to build 'mergepoint' information Using the walk cache gives use a better complexity we need to scale. There are further improvement we could make, but this is a good first step.