feat(engine): add story arc system for narrative-driven world

- Add StoryArc and StoryEventResult models
- Create story_arcs.py with progress tracking logic
- Implement two story arcs: civil_unrest and golden_age
- Progress updates based on faction power balance
- Trigger story events when progress reaches threshold
- Add 3 tests for story arc system

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

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

engine-python/app/models.py

@@ -62,6 +62,23 @@ class FactionEventResult(BaseModel):
     source_faction: Optional[str] = None  # "optimists" | "fearful"
 
 
+class StoryArc(BaseModel):
+    """剧情线"""
+    progress: float = Field(default=0.0, ge=0.0, le=1.0)
+    threshold: float = 1.0
+    active: bool = True
+    stage: int = 1  # 当前阶段
+    description: str = ""
+
+
+class StoryEventResult(BaseModel):
+    """剧情事件触发结果"""
+    triggered: bool = False
+    arc_id: Optional[str] = None
+    event_name: Optional[str] = None
+    description: Optional[str] = None
+
+
 class AgentState(BaseModel):
     emotion: Emotion = Emotion.CALM
     goal: str = ""
@@ -84,6 +101,7 @@ class WorldState(BaseModel):
     global_meter: GlobalMeter = Field(default_factory=GlobalMeter)
     world_effects: List[WorldEffect] = Field(default_factory=list)
     factions: Factions = Field(default_factory=Factions)
+    story_arcs: Dict[str, StoryArc] = Field(default_factory=dict)
 
 
 class Event(BaseModel):
@@ -120,3 +138,4 @@ class StepResponse(BaseModel):
     actions: List[Action]
     global_event: GlobalEventResult = Field(default_factory=GlobalEventResult)
     triggered_faction_event: FactionEventResult = Field(default_factory=FactionEventResult)
+    story_event: StoryEventResult = Field(default_factory=StoryEventResult)