- Add map_engagement_test.go with tests for:
- Win prob dependency in map engagement (lead changes counted)
- Critical moments dependency in engagement score
- Empty/nil replay handling
- Complete ComputeWinProbability + SetWinProbability flow
This confirms the existing implementation already correctly:
- Computes win probability via Monte Carlo rollout (100 iterations)
- Sets win_prob and critical_moments on replay before serialization
- Calculates map engagement score from win_prob_crossings and critical_moments
- Writes engagement score to maps table via UpdateMapEngagement
Task: bf-qps
