feat(nadia): Implement percentage function in calculator.py
Outcome: done
Changed: src/calculator.py:1 (import Union), src/calculator.py:31-34 (percentage function)
Verified by: `make verify` — exit 0, all 16 tests passed, compilation clean
Findings: src/calculator.py:31-34 — percentage(value: Union[int, float], total: Union[int, float]) -> float added, returning (value/total)*100, raising ValueError on total==0
Open: none
Confidence: high — all acceptance criteria met, verification passed
Typosaurus-Run: ff52a86851934fd293e2f4493c5a9a46
Typosaurus-Node: baf30844321745fb8a1aabe91a33823b
Typosaurus-Agent: @nadia
Refs: #16
typosaurus
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