feat: Add Binary addition algorithm in C++

This program adds two binary numbers given as strings and returns their sum as a binary string.

About

This algorithm solves the problem of adding two binary numbers without using built-in base conversion functions.
It is based on LeetCode Problem 67 — Add Binary.

How it works

1. Traverse both strings from right to left (least significant bit first).
2. Add corresponding bits plus the carry.
3. Append (sum % 2) to the result and update carry = sum / 2.
4. Reverse the final result to get the correct binary sum.

Example

Input: a = "1010", b = "1011"
Output: "10101"

Reference

https://en.wikipedia.org/wiki/Binary_arithmetic

✅ Checklist

• I have read CONTRIBUTING.md.
• This pull request is all my own work — I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file.
• All new C++ files are placed inside an existing directory.
• All filenames are lowercase characters with underscores (no spaces or dashes).
• All functions and variable names follow C++ naming conventions (snake_case or camelCase).
• All function parameters and return values are clearly defined.
• All new algorithms include at least one URL that points to Wikipedia or another explanation.