Minor API inconsistency: Create{Unit,Feature} facing vs heading

* `Spring.CreateUnit` accepts a facing ([0; 3] cardinal direction)
* `Spring.CreateFeature` accepts a heading ([-32768; 32767] arbitrary direction), but then snaps it to a facing (i.e. if you pass a diagonal heading it takes the nearest cardinal direction regardless)

Ideally they would accept it in the same unit but I'm not sure how to approach it in a back-compatible way. Very low importance anyway.

Start by comparing the exposed Spring.CreateUnit and Spring.CreateFeature APIs and how each interprets direction values. Investigate the compatibility constraints around their existing facing and heading ranges. The work is done when a consistent direction unit is defined with a documented, backward-compatible migration or behavior.

Written by the indexing model from the issue text.