feat: add count_bits algorithm with doctests

[NA-V10]

Summary

This PR adds a new bit manipulation algorithm count_bits that returns the
number of set bits (1s) in a non-negative integer.

Highlights

• Pure Python bitwise implementation
• Includes doctests following project standards
• Validates against negative input
• No external dependencies

Checklist

• Doctests added
• Code follows repository format
• No breaking changes

Thank you!