Add pivot integer algorithm in pivot_integer.cpp

[Lumowhisp]

Implement an algorithm to find the pivot integer where the sum of numbers from 1 to x equals the sum from x to n.
The function returns the pivot integer or -1 if no pivot exists.

Description of Change

This PR adds a new implementation of the Pivot Integer problem inside the math directory.
The algorithm determines whether there exists an integer x such that:

1 + 2 + … + x = x + (x+1) + … + n

Using the mathematical property that a pivot exists only when the total sum of the first n natural numbers is a perfect square, the solution computes the pivot in O(1) time.
A small demonstration main() function is included for testing.

Checklist
• Added description of change
• File name follows guidelines (pivot_integer.cpp)
• Added test/example in main(), test passes
• Added clear documentation following Doxygen style
• Added relevant inline comments
• PR title follows semantic commit guidelines
• Searched for duplicates
• I acknowledge that all my contributions are made under the project’s license

Notes

Adds a clean, optimized, and well-documented math utility for computing the pivot integer.