Finite sized battle arenas with retreat option

Like in SE3. Give the battle arena a size big enough for reasonable sized fleets to be spread out outside of weapons range. Ships should be placed in groups based on which direction they moved into the sector from.

If a ship flees outside the battle arena, it retreats to the neighboring sector, consuming one strategic move. Retreat is not allowed if the ship has no strategic moves remaining; the ship will be confined to the arena in that case. Diagonal moves might be kind of tricky to detect.

We might need to adjust the ship movement logic in battle to prevent ships from inadvertently retreating.

You cannot retreat outside the edge of a star system. Perhaps we could allow retreating via warp points. Perhaps we could also prevent retreat into sectors with enemies, but what if they are cloaked?

This could potentially be a mod option which can be toggled in Settings.txt; perhaps also the arena size and distance between fleets could be set there as well.