Fix null reference error in weapon system collision observer

Fixed TypeError when accessing collision observable on disposed physics bodies.
The error occurred in two places:
1. Inside collision callback when trying to remove observer after hit
2. In timeout cleanup when projectile disposed before 2-second timer

Added defensive null checks and try-catch blocks in both locations to handle
race conditions where physics bodies are disposed during collision handling.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/weaponSystem.ts

@@ -129,20 +129,34 @@ export class WeaponSystem {
                 hitRecorded = true;
 
                 // Remove collision observer after first hit
-                if (collisionObserver) {
+                if (collisionObserver && ammoAggregate.body) {
+                    try {
                         ammoAggregate.body.getCollisionObservable().remove(collisionObserver);
+                    } catch (e) {
+                        // Body may have been disposed during collision handling, ignore
+                    }
                 }
             }
         });
 
         // Auto-dispose after 2 seconds
         window.setTimeout(() => {
-            // Clean up collision observer
-            if (collisionObserver) {
+            // Clean up collision observer if body still exists
+            if (collisionObserver && ammoAggregate.body) {
+                try {
                     ammoAggregate.body.getCollisionObservable().remove(collisionObserver);
+                } catch (e) {
+                    // Body may have already been disposed, ignore error
                 }
+            }
+
+            // Dispose if not already disposed
+            try {
                 ammoAggregate.dispose();
                 ammo.dispose();
+            } catch (e) {
+                // Already disposed, ignore
+            }
         }, 2000);
     }