fix(risk-metrics): zero-volatility Sharpe is -inf, which serializes to null and crashes the renderer #230

On main (d845531), RiskCalculator._calculate_sharpe divides by returns.std(). A flat price series gives std == 0 and a Sharpe of -inf. Pydantic's JSON output turns -inf into null, and the risk renderer then raises TypeError on the null.

Found by the second-model review of the comment-sweep analysis branch, which reproduced it on both the base and the branch. The branch names the outcome (_zero_risk_sharpe) but keeps the -inf value.

Fix: define an explicit contract for an unavailable or unbounded ratio, for example None with a reason field, and test its JSON round trip through every consumer.