[FR] BuildRange3D Y stretch

I am using builders with buildRange3D = true
I am satisfied with this behaviour, but at some point it may be needed to stretch the sphere by Y coordinate to enable building on some important large cliffs, purely from gameplay perspective. For example, one map I design very large cliffs but I want my builders able to build in certain places up high right on the edge of their build range.
I think it will be useful to have an Y coordinate scale for a unitdef (`buildRangeHeightMod`), similarly to what we have for the weapons

Start by tracing how builders use buildRange3D and how weapon range modifiers are represented on a unitdef. Define the expected behavior for buildRangeHeightMod, including Y scaling on large cliffs, and verify that builder placement respects the configured height range.

Written by the indexing model from the issue text.