Add ReturnOnInvestment to math

[InukaWijerathna]

Description

Adds ReturnOnInvestment to the math package.

ROI is a straightforward but widely used financial calculation that measures investment profitability:

ROI = (Gain - Cost) / Cost × 100

What's included

• math/returnoninvestment.go — returns (float64, error), following Go idioms
• math/returnoninvestment_test.go — table-driven tests covering all cases including error paths

Checklist

• Follows Go conventions (exported function, error return, package-level test)
• Table-driven tests with descriptive names
• Returns error (not panic) for invalid input

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

[codacy-production]

Not up to standards ⛔

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[InukaWijerathna]

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