Add return on investment to math

[InukaWijerathna]

Description

Adds return_on_investment to the math/ directory.

ROI is a basic but important financial calculation that tells you how profitable an investment was:

ROI = (Gain - Cost) / Cost × 100

What's included

• math/return_on_investment.c — implementation with Doxygen-style docs, assert-based self-tests, and graceful error handling for invalid input

Checklist

• Follows the code style of the project (Doxygen comments, assert tests, main() test runner)
• Handles invalid input (zero/negative cost) with stderr message and sentinel return value
• Self-contained with all tests in main()

Reference: https://www.investopedia.com/terms/r/returnoninvestment.asp

[InukaWijerathna]

Closing this for now — needs more work on my end before it's ready.

[InukaWijerathna]

Hi! Just checking in to see if there's any update on this PR — happy to make any changes needed to get it merged. Thanks for your time!