Clarify AirAssault flight plan method name.

`engagement_distance` is used elsewhere to mean commit distance, so this
looked like a bug when I stumbled across it. Rename it to be more
explicit.

game/ato/flightplans/airassault.py

@@ -63,8 +63,7 @@ class AirAssaultFlightPlan(StandardFlightPlan[AirAssaultLayout]):
         return None
 
     @property
-    def engagement_distance(self) -> Distance:
-        # The radius of the WaypointZone created at the target location
+    def ctld_target_zone_radius(self) -> Distance:
         return meters(2500)
 
     @property

game/missiongenerator/logisticsgenerator.py

@@ -46,7 +46,7 @@ class LogisticsGenerator:
             target_zone = f"{self.group.name}TARGET_ZONE"
             self.mission.triggers.add_triggerzone(
                 self.flight.flight_plan.layout.target.position,
-                self.flight.flight_plan.engagement_distance.meters,
+                self.flight.flight_plan.ctld_target_zone_radius.meters,
                 False,
                 target_zone,
             )