add example tests for damage

src/value-objects/Level.ts

@@ -31,4 +31,12 @@ export class Level {
   diff(target: Level): number {
     return this.value - target.value;
   }
+
+  /** Damage threshold to reach the given level.
+   *  Level 1→2: 1000, Level 2→3: 3000, Level 3→4: 6000, etc.
+   *  Formula: 1000 * N * (N+1) / 2
+   */
+  static damageThresholdForLevel(level: number): number {
+    return 1000 * level * (level + 1) / 2;
+  }
 }