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.
This alters the DEAD task planning to be the *least* preferred task, but
prevents other tasks from being planned unless they are excepted to be
clear of air defenses first. Even so, missions are a guaranteed success
so those other missions will still get SEAD escorts if there's potential
for a SAM in the area.
This means that air defenses that are not protecting a more useful
target (like a convoy, armor column, building, etc) will no longer be
considered by the mission planner. This isn't *quite* right since we
currently only check the target area for air defenses rather than the
entire flight plan, so there's a chance that we ignore IADS that have
threatened ingress points (though that's mostly solved by the flight
plan layout).
This also is still slightly limited because it's not checking for
aircraft availability at this stage yet, so we may aggressively plan
missions that we should be skipping unless we can guarantee that the
DEAD mission was planned. However, that's not new behavior.
An HTN (https://en.wikipedia.org/wiki/Hierarchical_task_network) is
similar to a decision tree, but it is able to reset to an earlier stage
if a subtask fails and tasks are able to account for the changes in
world state caused by earlier tasks.
Currently this just uses exactly the same strategy as before so we can
prove the system, but it should make it simpler to improve on task
planning.
