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/frontlinestancetask.py

@@ -0,0 +1,75 @@
+from __future__ import annotations
+
+import math
+from abc import ABC, abstractmethod
+from typing import TYPE_CHECKING
+
+from game.commander.tasks.theatercommandertask import TheaterCommanderTask
+from game.commander.theaterstate import TheaterState
+from game.profiling import MultiEventTracer
+from game.theater import FrontLine
+from gen.ground_forces.combat_stance import CombatStance
+
+if TYPE_CHECKING:
+    from gen.flights.ai_flight_planner import CoalitionMissionPlanner
+
+
+class FrontLineStanceTask(TheaterCommanderTask, ABC):
+    def __init__(self, front_line: FrontLine, player: bool) -> None:
+        self.front_line = front_line
+        self.friendly_cp = self.front_line.control_point_friendly_to(player)
+        self.enemy_cp = self.front_line.control_point_hostile_to(player)
+
+    @property
+    @abstractmethod
+    def stance(self) -> CombatStance:
+        ...
+
+    @staticmethod
+    def management_allowed(state: TheaterState) -> bool:
+        return not state.player or state.stance_automation_enabled
+
+    def better_stance_already_set(self, state: TheaterState) -> bool:
+        current_stance = state.front_line_stances[self.front_line]
+        if current_stance is None:
+            return False
+        preference = (
+            CombatStance.RETREAT,
+            CombatStance.DEFENSIVE,
+            CombatStance.AMBUSH,
+            CombatStance.AGGRESSIVE,
+            CombatStance.ELIMINATION,
+            CombatStance.BREAKTHROUGH,
+        )
+        current_rating = preference.index(current_stance)
+        new_rating = preference.index(self.stance)
+        return current_rating >= new_rating
+
+    @property
+    @abstractmethod
+    def have_sufficient_front_line_advantage(self) -> bool:
+        ...
+
+    @property
+    def ground_force_balance(self) -> float:
+        # TODO: Planned CAS missions should reduce the expected opposing force size.
+        friendly_forces = self.friendly_cp.deployable_front_line_units
+        enemy_forces = self.enemy_cp.deployable_front_line_units
+        if enemy_forces == 0:
+            return math.inf
+        return friendly_forces / enemy_forces
+
+    def preconditions_met(self, state: TheaterState) -> bool:
+        if not self.management_allowed(state):
+            return False
+        if self.better_stance_already_set(state):
+            return False
+        return self.have_sufficient_front_line_advantage
+
+    def apply_effects(self, state: TheaterState) -> None:
+        state.front_line_stances[self.front_line] = self.stance
+
+    def execute(
+        self, mission_planner: CoalitionMissionPlanner, tracer: MultiEventTracer
+    ) -> None:
+        self.friendly_cp.stances[self.enemy_cp.id] = self.stance