Forecast multiple available outputs together

Up to 20 independent forecast reads in one request. Each item uses the getForecast input and preserves its native target, units, horizon and issuance. Results retain request ids and per-output status, including failures and quota retry times. Each item uses the same authentication, entitlement, rate limit and metering as a single read. Availability comes from forecast-capabilities. This groups marginal forecasts; it does not generate correlated joint scenarios.