feat: add quick sort algorithm

Closes #6 (closed) - Add quick sort algorithm with median-of-three pivot selection

Changes

• Implement quick_sort with median-of-three pivot selection
• Add comprehensive test coverage
• Update demo to include quick sort
• Time complexity: O(n log n) average, O(n²) worst
• Space complexity: O(log n)

Test Results

✅ All 50 tests passed

Files Changed

• main.py: Add quick_sort to demo
• sort.py: Implement quick_sort algorithm
• test_sort.py: Add quick_sort tests