Let the TheaterCommander manage front line stance.

This improves the AI behavior by choosing the stances non-randomly:

* Breakthrough will be used if the base is expected to be capturable and
  the coalition outnumbers the enemy by 20%.
* Elimination will be used if the coalition has at least as many units
  as the enemy.
* Defensive will be used if the coalition has at least half as many
  units as the enemy.
* Retreat will be used if the coalition is significantly outnumbers.

This also exposes the option to the player.

game/operation/operation.py

@@ -397,6 +397,7 @@ class Operation:
                 player_gp,
                 enemy_gp,
                 player_cp.stances[enemy_cp.id],
+                enemy_cp.stances[player_cp.id],
                 cls.unit_map,
             )
             ground_conflict_gen.generate()