Added Problem 263

[Mantlemoose]

Summary

Please read CONTRIBUTING.md before submitting.

Type of Change

• New research problem
• [*] New algorithmic problem
• Bug fix
• Documentation update
• Other:

Testing

Checklist

• [*] Code follows the project structure and conventions
• [*] Self-review completed
• Documentation updated (if applicable)

CI Validation (for new problems)

When adding new problems, CI will automatically validate that your reference solution achieves score > 0.

• Algorithmic problems: Include reference.cpp in your problem directory
• Research problems: Include reference.py (or reference.cpp if language: cpp in config.yaml)

Add Problem 263, which was used in CALICO 2026 Spring, together with two reference solutions.