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/commander/tasks/primitive/breakthroughattack.py

@@ -0,0 +1,37 @@
+from __future__ import annotations
+
+from game.commander.tasks.frontlinestancetask import FrontLineStanceTask
+from game.commander.theaterstate import TheaterState
+from game.theater import ControlPoint
+from game.theater.theatergroundobject import VehicleGroupGroundObject
+from game.utils import meters
+from gen.ground_forces.combat_stance import CombatStance
+
+
+class BreakthroughAttack(FrontLineStanceTask):
+    @property
+    def stance(self) -> CombatStance:
+        return CombatStance.BREAKTHROUGH
+
+    @property
+    def have_sufficient_front_line_advantage(self) -> bool:
+        return self.ground_force_balance >= 1.2
+
+    @property
+    def opposing_garrisons_eliminated(self) -> bool:
+        # TODO: Should operate on TheaterState to account for BAIs planned this turn.
+        for tgo in self.enemy_cp.ground_objects:
+            if not isinstance(tgo, VehicleGroupGroundObject):
+                continue
+            if meters(tgo.distance_to(self.enemy_cp)) < ControlPoint.CAPTURE_DISTANCE:
+                return False
+        return True
+
+    def preconditions_met(self, state: TheaterState) -> bool:
+        if not super().preconditions_met(state):
+            return False
+        return self.opposing_garrisons_eliminated
+
+    def apply_effects(self, state: TheaterState) -> None:
+        super().apply_effects(state)
+        state.active_front_lines.remove(self.front_line)