feat(engine): 实现阵营投票系统

- 新增 Votes 模型，包含 optimists/fearful 计数和 voted_users 去重列表
- 扩展 Event 模型，添加可选 faction 字段支持投票事件
- 新增 voting.py 模块处理投票逻辑
- 投票规则：每用户每 tick 限投一次，下一 tick 生效
- 投票累加到 factions.power，达到 threshold 触发阵营技能
- 添加 5 个投票系统测试用例，全部 26 个测试通过

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

engine-python/tests/test_step.py

@@ -381,3 +381,103 @@ def test_story_event_in_response(client):
     assert "story_event" in data
     assert "triggered" in data["story_event"]
     assert "arc_id" in data["story_event"]
+
+
+# ==================== 投票系统测试 ====================
+
+def test_votes_exists_in_world_state(client):
+    """测试 world_state 包含 votes 字段"""
+    resp = client.post("/step", json={"events": []})
+    data = resp.json()
+
+    assert "votes" in data["world_state"]
+    votes = data["world_state"]["votes"]
+    assert "optimists" in votes
+    assert "fearful" in votes
+    assert "voted_users" in votes
+
+
+def test_vote_records_correctly(client):
+    """测试投票正确记录"""
+    events = [
+        {"type": "vote", "faction": "optimists", "text": "", "user": "user1", "ts": 1},
+        {"type": "vote", "faction": "fearful", "text": "", "user": "user2", "ts": 2},
+    ]
+    resp = client.post("/step", json={"events": events})
+    data = resp.json()
+
+    votes = data["world_state"]["votes"]
+    assert votes["optimists"] == 1
+    assert votes["fearful"] == 1
+    assert "user1" in votes["voted_users"]
+    assert "user2" in votes["voted_users"]
+
+
+def test_vote_user_deduplication(client):
+    """测试同一用户在同一 tick 只能投一次票"""
+    events = [
+        {"type": "vote", "faction": "optimists", "text": "", "user": "user1", "ts": 1},
+        {"type": "vote", "faction": "optimists", "text": "", "user": "user1", "ts": 2},
+        {"type": "vote", "faction": "fearful", "text": "", "user": "user1", "ts": 3},
+    ]
+    resp = client.post("/step", json={"events": events})
+    data = resp.json()
+
+    votes = data["world_state"]["votes"]
+    # 只计算第一次投票
+    assert votes["optimists"] == 1
+    assert votes["fearful"] == 0
+
+
+def test_votes_apply_to_faction_power_next_tick(client):
+    """测试投票在下一个 tick 累加到阵营能量"""
+    # 第一个 tick：投票
+    events = [
+        {"type": "vote", "faction": "optimists", "text": "", "user": "user1", "ts": 1},
+        {"type": "vote", "faction": "optimists", "text": "", "user": "user2", "ts": 2},
+        {"type": "vote", "faction": "fearful", "text": "", "user": "user3", "ts": 3},
+    ]
+    resp1 = client.post("/step", json={"events": events})
+    data1 = resp1.json()
+
+    # 记录第一个 tick 后的阵营能量
+    power1_opt = data1["world_state"]["factions"]["optimists"]["power"]
+    power1_fear = data1["world_state"]["factions"]["fearful"]["power"]
+
+    # 第二个 tick：投票应该生效
+    resp2 = client.post("/step", json={"events": []})
+    data2 = resp2.json()
+
+    power2_opt = data2["world_state"]["factions"]["optimists"]["power"]
+    power2_fear = data2["world_state"]["factions"]["fearful"]["power"]
+
+    # 验证投票已累加（+2 optimists, +1 fearful）
+    assert power2_opt >= power1_opt + 2
+    assert power2_fear >= power1_fear + 1
+
+    # 验证投票已清空
+    assert data2["world_state"]["votes"]["optimists"] == 0
+    assert data2["world_state"]["votes"]["fearful"] == 0
+
+
+def test_votes_trigger_faction_event(client):
+    """测试投票累积触发阵营技能"""
+    # 多轮投票累积能量直到触发
+    for i in range(5):
+        events = [
+            {"type": "vote", "faction": "optimists", "text": "", "user": f"user_{i}_a", "ts": i},
+            {"type": "vote", "faction": "optimists", "text": "", "user": f"user_{i}_b", "ts": i + 0.1},
+        ]
+        resp = client.post("/step", json={"events": events})
+
+    # 再投一轮让投票生效
+    resp = client.post("/step", json={"events": []})
+    data = resp.json()
+
+    # 验证阵营能量已累积（可能已触发并重置）
+    # 如果触发了，power 会重置为 0
+    factions = data["world_state"]["factions"]
+    triggered = data["triggered_faction_event"]["type"] is not None
+
+    # 要么触发了事件，要么能量在累积
+    assert triggered or factions["optimists"]["power"] > 0