feat(engine): add faction power system with event triggers

- Add FactionData model with power and members fields
- Add FactionEventResult model for faction event responses
- Add faction field to AgentState
- Implement faction classification based on emotion
- Add faction event triggers (festival/panic) when power >= 5
- Update StepResponse to include triggered_faction_event
- Add tests for new faction system

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

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

engine-python/app/state.py

@@ -1,6 +1,6 @@
 import json
 from pathlib import Path
-from .models import WorldState, AgentState, GlobalMeter, Stance
+from .models import WorldState, AgentState, GlobalMeter, Stance, Factions, FactionData
 
 STATE_FILE = Path(__file__).parent.parent / "state.json"
 
@@ -17,16 +17,22 @@ def get_default_state() -> WorldState:
                 goal="探索小镇",
                 memory=[],
                 stance=Stance(optimism=0.6, fear=0.4),
+                faction="neutral",
             ),
             "bob": AgentState(
                 emotion="calm",
                 goal="与人交流",
                 memory=[],
                 stance=Stance(optimism=0.4, fear=0.6),
+                faction="neutral",
             ),
         },
         events=[],
         global_meter=GlobalMeter(value=0, threshold=100, cooldown=0),
+        factions=Factions(
+            optimists=FactionData(power=0, members=[]),
+            fearful=FactionData(power=0, members=[])
+        ),
     )