feat(cards): implement retain keyword

tools/balance/sim-balance.mjs

@@ -212,7 +212,7 @@ export function simulateCombat(data, rng, stats) {
       else if (pc.powerEffect === 'energyPerTurn') energyBonus += pc.value;
       else if (pc.powerEffect === 'blockPerTurn') pBlock += pc.value;
     }
-    let energy = ENERGY + energyBonus; hand = []; draw(HAND_SIZE);
+    let energy = ENERGY + energyBonus; draw(HAND_SIZE);
     while (true) {
       const alive = aliveList();
       if (alive.length === 0) break;
@@ -230,7 +230,13 @@ export function simulateCombat(data, rng, stats) {
     let burn = 0;
     for (const hid of hand) { const hc = cards[hid]; if (hc && hc.endTurnDamage) burn += hc.endTurnDamage; }
     if (burn > 0) { pHp -= burn; if (pHp < 0) pHp = 0; }
-    discard.push(...hand); hand = [];
+    const kept = [];
+    for (const hid of hand) {
+      const hc = cards[hid];
+      if (hc?.retain === true) kept.push(hid);
+      else discard.push(hid);
+    }
+    hand = kept;
     if (pHp <= 0) return { win: false, turns, playerHpRemaining: 0 };
     // 플레이어 디버프 감소 — Lua EndPlayerTurn 동기화 (적 행동 전)
     if (pWeak > 0) pWeak--;

tools/balance/sim-balance.test.mjs

@@ -390,3 +390,18 @@ test("simulateCombat: sly discarded card resolves for free", () => {
   assert.equal(r.win, true);
   assert.equal(r.turns, 1);
 });
+
+test("simulateCombat: retain keeps card in hand across turns", () => {
+  const data = {
+    cards: {
+      Boost: { name: "Boost", cost: 3, kind: "Power", powerEffect: "energyPerTurn", value: 98 },
+      Hold: { name: "Hold", cost: 100, kind: "Attack", damage: 10, retain: true },
+      Blank: { name: "Blank", cost: 99, kind: "Skill", block: 0 },
+    },
+    starterDeck: ["Blank", "Blank", "Blank", "Blank", "Blank", "Boost", "Hold", "Blank", "Blank", "Blank"],
+    monsters: [{ name: "Dummy", maxHp: 10, intents: [{ kind: "Defend", value: 0 }] }],
+  };
+  const r = simulateCombat(data, () => 0.999999);
+  assert.equal(r.win, true);
+  assert.equal(r.turns, 2);
+});