perf: optimize factor calculation

[oozan]

Description

Optimizes factor calculation by examining divisor pairs only up to the square root of the input.

This reduces the running time from O(n) to O(sqrt(n)) and avoids allocating an intermediate array containing every integer up to the input. Results remain sorted, and perfect-square roots are not duplicated.

Testing

• Added exact-result coverage for one, composite numbers, a perfect square, and a prime
• Added invalid-input coverage
• Ran direct Node.js algorithm checks successfully
• Full formatting and test suites will run in GitHub CI

AI assistance was used to prepare this focused change. I reviewed and validated the implementation.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.93%. Comparing base (5c39e87) to head (edfe247).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1910      +/-   ##
==========================================
+ Coverage   85.91%   85.93%   +0.01%     
==========================================
  Files         379      379              
  Lines       19778    19798      +20     
  Branches     3016     3021       +5     
==========================================
+ Hits        16993    17013      +20     
  Misses       2785     2785              

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