Pull siege engine aggro on damage only

A siege engine switches to whoever damages it, player or pet, even when
it already has a target. Pre-roll melee retaliation, harmful-power
direct aggro (which also fires on misses) and Taunt no longer change a
siege engine's target. The attack timer is untouched, so a switched
engine still waits for its next cycle before firing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/engine/objects/Mob.java

@@ -1318,6 +1318,11 @@ public class Mob extends AbstractIntelligenceAgent implements Delayed {
         if (!ac.getObjectType().equals(GameObjectType.PlayerCharacter))
             return;
 
+        // siege engines only switch targets on damage, see AbstractCharacter.modifyHealth
+
+        if (this.isSiege())
+            return;
+
         if (this.getCombatTarget() == null)
             this.setCombatTarget(ac);