[integer] Implement Burnikel-Ziegler fast recursive division algorithm for BigUInt

[forfudan]

This PR implements the Burnikel-Ziegler fast recursive division algorithm which significantly improves the performance of large-number divisions (particularly when there are thousands of words). It achieves a realized time complexity of O(n^1.585) (3x time when digits are doubled) instead of the O(n^2) (4x time when digits are doubled).

Major Division Algorithm Improvements

1. Burnikel-Ziegler Algorithm Implementation

• New fast recursive division algorithm for large numbers
• Added floor_divide_burnikel_ziegler() function with cutoff at 64 words
• Implemented supporting functions:
  • floor_divide_two_by_one()
  • floor_divide_three_by_two()
  • floor_divide_three_by_two_uint32()
  • floor_divide_four_by_two_uint32()

2. Division Strategy Refactoring

• Renamed floor_divide_general() → floor_divide_school()
• Smart algorithm selection in floor_divide():
  • Small numbers (≤64 words): Use schoolbook division
  • Large numbers (>64 words): Use Burnikel-Ziegler algorithm
• Extracted normalization logic into calculate_normalization_factor() helper function

Benchmark and Testing Enhancements

3. Extended Test Coverage

• Increased test range from 2^16 to 2^18 words (262,144 words)
• Added validation to compare Mojo vs Python results
• Reduced iterations for large number tests (100 → 10)
• Added new test cases for extremely large divisions

4. Improved Benchmarking

• Extended Python string limit from 1M to 10M digits
• Enhanced logging with result validation
• Better error handling for mismatched results

Code Quality Improvements

5. Constructor Safety

• Fixed BigUInt constructor to handle empty word lists
• Added validation to prevent zero-length word arrays

6. Build Process

• Improved formatting command to target specific directories
• Better project organization

Performance Impact

The main achievement is dramatically improved performance for large number division:

• O(n²) → O(n^1.585) complexity reduction using Burnikel-Ziegler
• Automatic algorithm selection based on input size
• Maintains compatibility with existing API

This is a significant algorithmic improvement that should provide substantial speedups for large number operations while maintaining correctness for all input sizes.