feat: story 2 — Characters can Deal Damage to Characters

- Allium spec: damage-and-health.allium
- 5 fast-check properties: damage reduces health, health non-negative,
  death at zero, self-damage forbidden, dead cannot take damage
- Character.createWithHealth factory for testing with arbitrary health
- Character.dealDamage(target, damage) method

specs/damage-and-health.allium

@@ -0,0 +1,49 @@
+-- allium: 3
+
+-- allium: damage-and-health
+
+------------------------------------------------------------
+-- Rules
+------------------------------------------------------------
+
+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:
+        if target.health.value - damage <= 0:
+            target.status = dead
+        else:
+            target.status = alive
+}
+
+rule SelfDamageForbidden {
+    when: Character.dealDamage(attacker, target, damage)
+    requires: attacker.name = target.name
+    ensures:
+        target.health.value = target.health.value
+        target.status = target.status
+}
+
+rule DeadCannotTakeDamage {
+    when: Character.dealDamage(attacker, target, damage)
+    requires: target.status = dead
+    ensures:
+        target.health.value = target.health.value
+        target.status = dead
+}
+
+------------------------------------------------------------
+-- Invariants
+------------------------------------------------------------
+
+invariant HealthNonNegative {
+    for c in Characters:
+        c.health.value >= 0
+}
+
+invariant DeathAtZeroHealth {
+    for c in Characters:
+        c.health.value = 0 implies c.status = dead
+}