Add O(1) optimized solution for Project Euler Problem 6 #hacktoberfest

[KRiSHnA88H]

Description

This PR adds an O(1) optimized solution for Project Euler Problem 6 in C.
It uses the mathematical formulas:

• Sum of squares = n(n+1)(2n+1)/6
• Square of sum = (n(n+1)/2)^2

Changes

• Added new file o(1)sol.c in project_euler/problem_6/
• Provides a faster O(1) implementation compared to iterative methods
• Included comments and example usage for better understanding

Example

Input:
100

Output:
25164150

Notes

• Uses long long to avoid overflow for larger values of n
• Code is well-documented to support educational use

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