LeetCode 136: Single Number

## [LeetCode 136: Single Number](https://leetcode.com/problems/single-number/)

**Difficulty:** Easy
**Category:** Bit Manipulation

### Tasks
- [ ] Implement solution in `src/bit_manipulation/single_number.py`
- [ ] Verify tests pass: `pytest tests/test_single_number.py`
- [ ] Analyze time and space complexity