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/main.py

@@ -3,6 +3,7 @@ from .models import StepRequest, StepResponse
 from .state import load_state, save_state
 from .engine import process_events, generate_actions, check_and_trigger_global_event
 from .factions import check_and_trigger_faction_event
+from .story_arcs import update_story_progress, check_and_trigger_story_event
 
 app = FastAPI(title="AI Town Engine", version="0.1.0")
 
@@ -22,6 +23,12 @@ def step(request: StepRequest) -> StepResponse:
     # 检查并触发阵营事件
     faction_event_result = check_and_trigger_faction_event(state)
 
+    # 更新剧情进度
+    update_story_progress(state)
+
+    # 检查并触发剧情事件
+    story_event_result = check_and_trigger_story_event(state)
+
     # 生成 agent 行动
     actions = generate_actions(state)
 
@@ -32,7 +39,8 @@ def step(request: StepRequest) -> StepResponse:
         world_state=state,
         actions=actions,
         global_event=global_event_result,
-        triggered_faction_event=faction_event_result
+        triggered_faction_event=faction_event_result,
+        story_event=story_event_result
     )