feat: 添加观点传播系统

- 新增 Stance 数据结构 (optimism/fear)
- 情绪影响 stance (happy→乐观, anxious→恐惧)
- 实现 apply_social_influence 社交影响函数
- 确定性随机选择接触对象
- 单次变化限制 ±0.1

🤖 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

@@ -229,3 +229,69 @@ def test_opinion_cleared_when_no_effect(client):
     # 无世界事件时，opinion 应为 None
     for agent_id, agent in data["world_state"]["agents"].items():
         assert agent["opinion"] is None
+
+
+def test_stance_exists_in_agent(client):
+    """测试 agent 包含 stance 字段"""
+    resp = client.post("/step", json={"events": []})
+    data = resp.json()
+
+    for agent_id, agent in data["world_state"]["agents"].items():
+        assert "stance" in agent
+        assert "optimism" in agent["stance"]
+        assert "fear" in agent["stance"]
+        assert 0.0 <= agent["stance"]["optimism"] <= 1.0
+        assert 0.0 <= agent["stance"]["fear"] <= 1.0
+
+
+def test_stance_changes_on_world_event(client):
+    """测试世界事件触发时 stance 变化"""
+    # 记录初始 stance
+    resp1 = client.post("/step", json={"events": []})
+    initial_stances = {
+        aid: agent["stance"].copy()
+        for aid, agent in resp1.json()["world_state"]["agents"].items()
+    }
+
+    # 触发世界事件
+    events = [
+        {"type": "chaos", "text": f"chaos{i}", "user": "test", "ts": i}
+        for i in range(20)
+    ]
+    resp2 = client.post("/step", json={"events": events})
+    data = resp2.json()
+
+    # 验证事件触发
+    assert data["global_event"]["triggered"] is True
+
+    # stance 应该有变化（由于情绪和社交影响）
+    for aid, agent in data["world_state"]["agents"].items():
+        # stance 值仍在有效范围内
+        assert 0.0 <= agent["stance"]["optimism"] <= 1.0
+        assert 0.0 <= agent["stance"]["fear"] <= 1.0
+
+
+def test_social_influence_deterministic(client):
+    """测试社交影响是确定性的（同输入同输出）"""
+    events = [{"type": "comment", "text": "hello", "user": "test", "ts": 1}]
+
+    # 第一次执行
+    save_state(get_default_state())
+    resp1 = client.post("/step", json={"events": events})
+    stances1 = {
+        aid: agent["stance"]
+        for aid, agent in resp1.json()["world_state"]["agents"].items()
+    }
+
+    # 重置并再次执行
+    save_state(get_default_state())
+    resp2 = client.post("/step", json={"events": events})
+    stances2 = {
+        aid: agent["stance"]
+        for aid, agent in resp2.json()["world_state"]["agents"].items()
+    }
+
+    # 结果应该相同
+    for aid in stances1:
+        assert stances1[aid]["optimism"] == stances2[aid]["optimism"]
+        assert stances1[aid]["fear"] == stances2[aid]["fear"]