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/app/models.py

@@ -44,6 +44,13 @@ class Stance(BaseModel):
     fear: float = Field(default=0.5, ge=0.0, le=1.0)
 
 
+class Factions(BaseModel):
+    """派系分布"""
+    optimists: int = 0
+    fearful: int = 0
+    neutral: int = 0
+
+
 class AgentState(BaseModel):
     emotion: Emotion = Emotion.CALM
     goal: str = ""
@@ -63,6 +70,7 @@ class WorldState(BaseModel):
     events: List[str] = Field(default_factory=list)
     global_meter: GlobalMeter = Field(default_factory=GlobalMeter)
     world_effects: List[WorldEffect] = Field(default_factory=list)
+    factions: Factions = Field(default_factory=Factions)
 
 
 class Event(BaseModel):