feat: add rightmost_set_bit function

[AliAlimohammadi]

Description

Adds rightmost_set_bit function that finds the 1-based position of the rightmost (least significant) set bit in a positive integer.

Algorithm

Uses n & -n to isolate the rightmost set bit, then trailing_zeros() + 1 for position.

Example: For n = 18 (binary: 10010):

• n & -n = 2 (binary: 10)
• trailing_zeros(2) = 1
• position = 1 + 1 = 2 ✓

Features

• ✅ $O(1)$ time/space complexity
• ✅ Hardware-optimized (uses BSF/CTZ instruction)
• ✅ Two implementations (trailing_zeros and log2)
• ✅ Comprehensive error handling
• ✅ 15+ test cases

Examples

rightmost_set_bit(18) // Ok(2)  - binary: 10010
rightmost_set_bit(12) // Ok(3)  - binary: 1100
rightmost_set_bit(5)  // Ok(1)  - binary: 101
rightmost_set_bit(0)  // Err    - no set bits

Testing

All tests pass including:

• Powers of two
• Odd numbers (always position 1)
• Even numbers
• Edge cases (zero, negatives)
• Large numbers
• Both implementations match

Checklist

• Follows Rust style guidelines
• Passes clippy lints
• All tests pass
• Documentation with examples
• Properly exported

[AliAlimohammadi]

@siriak, this is ready to be merged.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.72%. Comparing base (7a261d7) to head (4798d6c).

Files with missing lines	Patch %	Lines
src/bit_manipulation/rightmost_set_bit.rs	98.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #975   +/-   ##
=======================================
  Coverage   95.71%   95.72%           
=======================================
  Files         346      347    +1     
  Lines       22608    22683   +75     
=======================================
+ Hits        21639    21713   +74     
- Misses        969      970    +1     

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