derive threat cost dynamically

src/lib/config/ShipsConfig.h

@@ -16,13 +16,6 @@ struct ShipSchematic
     double productionTimeSeconds;
 };
 
-// Wave scheduling cost (REQ-WAV-THREAT-COST). Ships with cost_formula that
-// always evaluates to 0 are ineligible as wave picks.
-struct ShipThreat
-{
-    Formula costFormula;
-};
-
 struct ShipHealth
 {
     Formula hpFormula;  // REQ-SHP-STATS
@@ -55,7 +48,6 @@ struct ShipDef
     std::vector<std::string> layout;
 
     ShipSchematic schematic;
-    ShipThreat threat;
     ShipHealth health;
     ShipMovement movement;
     ShipSensor sensor;