Add ReturnOnInvestment to maths

[InukaWijerathna]

Description

Adds ReturnOnInvestment to the maths/ directory.

Return on Investment (ROI) is a foundational financial metric that quantifies investment profitability as a percentage:

ROI = (Gain - Cost) / Cost × 100

What's included

• maths/return_on_investment.rb — Ruby class with frozen string literal, raises ArgumentError for invalid cost

Checklist

• Follows the code style of the project (class with self.call, frozen_string_literal, guard clause)
• Raises ArgumentError for non-positive cost
• Uses .to_f to ensure float division

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

[InukaWijerathna]

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