refactor(story2): immutable dealDamage, reference guard, negative damage validation, spec precision

- dealDamage returns new Character instead of mutating in-place
- SelfDamageForbidden uses reference equality (this === target)
- Negative damage throws at the boundary
- Removed duplicate Health.maxHealthForLevel (Level.ts is source of truth)
- Allium spec uses max(0, ...) for health floor precision
- New property: NegativeDamageForbidden (11 total properties)

specs/damage-and-health.allium

@@ -10,9 +10,9 @@ rule DamageReducesHealth {
     when: Character.dealDamage(attacker, target, damage)
     requires: attacker.name != target.name
     requires: target.status = alive
-    ensures: target.health.value = target.health.value - damage
+    ensures: target.health.value = max(0, target.health.value - damage)
     ensures:
-        if target.health.value - damage <= 0:
+        if max(0, target.health.value - damage) = 0:
             target.status = dead
         else:
             target.status = alive

src/Character.ts

@@ -24,9 +24,11 @@ export interface CharacterCtorWithHealth {
 
 export class Character {
   #state: CharacterState;
+  readonly #name: string;
 
   private constructor(state: CharacterState) {
     this.#state = state;
+    this.#name = state.name;
   }
 
   /** Create a new character with default health (1000) and alive status. */
@@ -42,7 +44,7 @@ export class Character {
   }
 
   get name(): string {
-    return this.#state.name;
+    return this.#name;
   }
 
   get health(): Health {
@@ -61,21 +63,22 @@ export class Character {
     return this.#state.factions;
   }
 
-  /** Deal damage to another character. Mutates target in place. */
-  dealDamage(target: Character, damage: number): void {
-    // Self-damage is forbidden
-    if (this.name === target.name) return;
+  /**
+   * Deal damage to another character. Returns a new Character with updated state.
+   * Does not mutate the attacker or the original target reference.
+   */
+  dealDamage(target: Character, damage: number): Character {
+    // Self-damage is forbidden — use reference equality, not name
+    if (this === target) return target;
     // Dead characters cannot take damage
-    if (target.status.kind === 'dead') return;
+    if (target.status.kind === 'dead') return target;
+    // Negative damage is invalid
+    if (damage < 0) throw new Error(`Damage must be non-negative, got ${damage}`);
     // Reduce health
     const newHealth = target.health.sub(damage);
     const newStatus = newHealth.value === 0 ? StatusDead : StatusAlive;
-    target.#state = new CharacterState(
-      target.name,
-      newHealth,
-      newStatus,
-      target.level,
-      target.factions,
+    return new Character(
+      new CharacterState(target.name, newHealth, newStatus, target.level, target.factions),
     );
   }
 }

src/Health.ts

@@ -11,10 +11,6 @@ export class Health {
     this.#value = n;
   }
 
-  static maxHealthForLevel(level: number): number {
-    return level >= 6 ? 1500 : 1000;
-  }
-
   static create(n: number): Health {
     if (n < 0) {
       throw new Error(`Health cannot be negative, got ${n}`);

src/damage-and-health.spec.ts

@@ -18,8 +18,8 @@ describe('DamageAndHealth', () => {
               health,
             });
             const expected = Math.max(0, health - damage);
-            attacker.dealDamage(target, damage);
-            return target.health.value === expected;
+            const result = attacker.dealDamage(target, damage);
+            return result.health.value === expected;
           },
         ),
       );
@@ -39,8 +39,8 @@ describe('DamageAndHealth', () => {
               level: Level.create(1),
               health,
             });
-            attacker.dealDamage(target, damage);
-            return target.health.value >= 0;
+            const result = attacker.dealDamage(target, damage);
+            return result.health.value >= 0;
           },
         ),
       );
@@ -60,12 +60,12 @@ describe('DamageAndHealth', () => {
               level: Level.create(1),
               health,
             });
-            attacker.dealDamage(target, damage);
+            const result = attacker.dealDamage(target, damage);
             const expected = Math.max(0, health - damage);
             if (expected === 0) {
-              return target.status.kind === 'dead';
+              return result.status.kind === 'dead';
             }
-            return target.status.kind === 'alive';
+            return result.status.kind === 'alive';
           },
         ),
       );
@@ -73,7 +73,7 @@ describe('DamageAndHealth', () => {
   });
 
   describe('SelfDamageForbidden', () => {
-    it('property: a character cannot deal damage to itself — health unchanged', () => {
+    it('property: a character cannot deal damage to itself — returns same reference, state unchanged', () => {
       fc.assert(
         fc.property(
           fc.integer({ min: 0, max: 1000 }),
@@ -82,8 +82,13 @@ describe('DamageAndHealth', () => {
             const c = Character.createWithHealth({ name: 'hero', level: Level.create(1), health });
             const healthBefore = c.health.value;
             const statusBefore = c.status.kind;
-            c.dealDamage(c, damage);
-            return c.health.value === healthBefore && c.status.kind === statusBefore;
+            const result = c.dealDamage(c, damage);
+            // Should return the same reference
+            return (
+              result === c &&
+              result.health.value === healthBefore &&
+              result.status.kind === statusBefore
+            );
           },
         ),
       );
@@ -91,18 +96,40 @@ describe('DamageAndHealth', () => {
   });
 
   describe('DeadCannotTakeDamage', () => {
-    it('property: dead characters cannot take damage — state unchanged', () => {
+    it('property: dead characters cannot take damage — returns same reference, state unchanged', () => {
       fc.assert(
         fc.property(fc.integer({ min: 0, max: 10000 }), (damage) => {
           const attacker = Character.create({ name: 'attacker', level: Level.create(1) });
           const target = Character.create({ name: 'target', level: Level.create(1) });
-          // Kill the target first
-          attacker.dealDamage(target, 10000);
-          const healthBefore = target.health.value;
-          const statusBefore = target.status.kind;
-          // Then try to deal more damage
-          attacker.dealDamage(target, damage);
-          return target.health.value === healthBefore && target.status.kind === statusBefore;
+          // Kill the target first — capture the returned (dead) character
+          const deadTarget = attacker.dealDamage(target, 10000);
+          const healthBefore = deadTarget.health.value;
+          const statusBefore = deadTarget.status.kind;
+          // Then try to deal more damage to the dead character
+          const result = attacker.dealDamage(deadTarget, damage);
+          return (
+            result === deadTarget &&
+            result.health.value === healthBefore &&
+            result.status.kind === statusBefore
+          );
+        }),
+      );
+    });
+  });
+
+  describe('NegativeDamageForbidden', () => {
+    it('property: negative damage throws an error', () => {
+      fc.assert(
+        fc.property(fc.integer({ min: -10000, max: -1 }), (negativeDamage) => {
+          const attacker = Character.create({ name: 'attacker', level: Level.create(1) });
+          const target = Character.create({ name: 'target', level: Level.create(1) });
+          let threw = false;
+          try {
+            attacker.dealDamage(target, negativeDamage);
+          } catch {
+            threw = true;
+          }
+          return threw;
         }),
       );
     });