feat(engine): add faction system for social emergence

- Add Factions model (optimists/fearful/neutral)
- Implement classify_faction() based on stance thresholds
- Add update_factions() to track faction distribution
- Add apply_faction_influence() for faction→mood feedback
- Integrate faction system into tick flow

🤖 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

@@ -295,3 +295,27 @@ def test_social_influence_deterministic(client):
     for aid in stances1:
         assert stances1[aid]["optimism"] == stances2[aid]["optimism"]
         assert stances1[aid]["fear"] == stances2[aid]["fear"]
+
+
+def test_factions_exists_in_world_state(client):
+    """测试 world_state 包含 factions 字段"""
+    resp = client.post("/step", json={"events": []})
+    data = resp.json()
+
+    assert "factions" in data["world_state"]
+    factions = data["world_state"]["factions"]
+    assert "optimists" in factions
+    assert "fearful" in factions
+    assert "neutral" in factions
+
+
+def test_factions_count_matches_agents(client):
+    """测试派系总数等于 agent 数量"""
+    resp = client.post("/step", json={"events": []})
+    data = resp.json()
+
+    factions = data["world_state"]["factions"]
+    total = factions["optimists"] + factions["fearful"] + factions["neutral"]
+    agent_count = len(data["world_state"]["agents"])
+
+    assert total == agent_count