Refactor duplicated code into subroutines

Future enhancement: Identify and refactor duplicated code blocks of more than 6 lines into reusable subroutines.

Current issues likely include:
- Player input handling patterns (joy0left/joy1left, joy0right/joy1right, etc.)
- Character selection logic for different players
- Collision detection patterns
- Sprite positioning and rendering code
- Animation state handling

Process:
1. Scan codebase for duplicated code blocks > 6 lines
2. Identify common patterns and functionality
3. Create parameterized subroutines where possible
4. Replace duplicated code with subroutine calls
5. Ensure subroutines are properly documented

Benefits:
- Reduced code duplication
- Easier maintenance and bug fixes
- More consistent behavior
- Smaller ROM size

Priority: Future enhancement after core build issues are resolved.
Note: This requires careful analysis to ensure subroutines don't introduce performance issues.