- Add DamageCalculationResult model for transparent damage breakdown
- Implement _calculate_attack_damage with W/R modifiers (additive/multiplicative)
- Add _execute_attack_effect for future effect system integration
- Add _build_attack_message for detailed damage breakdown in messages
- Update _execute_attack to use new calculation pipeline
- Bulbasaur now properly weak to Lightning in walkthrough demo
New features:
- Weakness applies bonus damage (additive +X or multiplicative xN)
- Resistance reduces damage (minimum 0)
- State changes include weakness/resistance details for UI
- Messages show damage breakdown (e.g. 'base 10 +20 weakness')
Tests: 7 new tests covering additive/multiplicative W/R, type matching,
minimum damage floor, knockout triggers, and state change details