Added distance-based combat retreat

I made retreat depend on distance from the active enemies. When every living squad member moves beyond their pursuit reach, combat can end instead of a previous provocation holding the squad in it indefinitely.

The default reach is sixteen tiles, or 26.4 metres; an opening long-range shot extends the appropriate pursuit range. Damage remains saved, and approaching again can start another encounter. The clips use an initialized isolated encounter and real movement across its boundary.

“A squad can disengage by moving beyond the active enemies' pursuit reach.”