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/state.py

@@ -1,6 +1,7 @@
 import json
 from pathlib import Path
 from .models import WorldState, AgentState, GlobalMeter, Stance, Factions, FactionData
+from .story_arcs import get_default_story_arcs
 
 STATE_FILE = Path(__file__).parent.parent / "state.json"
 
@@ -33,6 +34,7 @@ def get_default_state() -> WorldState:
             optimists=FactionData(power=0, members=[]),
             fearful=FactionData(power=0, members=[])
         ),
+        story_arcs=get_default_story_arcs(),
     )