refactor: optimize Single class with O(1) algorithms and validation

[vboursinos]

Summary

Replaces inefficient O(n) implementations in Single class with O(1) mathematical formulas and adds input validation.

Changes

• Replace array-based sumRange with arithmetic series formula (O(n) → O(1))
• Fix maxArray to handle negative numbers correctly (was incorrectly initializing max to 0)
• Optimize sumModulus using arithmetic series instead of Vector iteration
• Add comprehensive input validation with appropriate exceptions
• Improve Javadoc documentation with complexity analysis and examples

Testing

• Verify sumRange handles edge cases (n=0, n=1, negative n)
• Confirm maxArray works with arrays containing only negative numbers
• Test sumModulus with various modulus values including negative
• Validate exception throwing for invalid inputs

Notes

Breaking change: maxArray now throws IllegalArgumentException for null arrays instead of NullPointerException. sumModulus and sumRange now validate inputs and throw exceptions for invalid values.