Implement conflict detection #98
Description
Add comprehensive conflict detection to overlay builder with detailed logging and reporting.
Track which mods contribute each file, detect conflicts, and determine winners based on priority rules.
See implementation plan in docs/overlay-implementation-plan.md Phase 4.
Acceptance Criteria:
- Conflicts detected correctly
- Winner determined by documented priority rules
- Clear logging with full context
- OverlayBuildResult includes conflict information
- Unit tests for priority resolution
Related: Epic #85