Validate hourly forecast time bounds

docs/internal/weather-data.md

@@ -45,6 +45,10 @@ marked missing only when the adapter's missing-source policy treats the
 response or parsing failure as unavailable. The policy itself belongs to the
 [configuration reference](../config.md).
 
+Accepted hourly forecast periods always have nonzero start and end times, with
+the end after the start. Collection rejects a required hourly product that does
+not meet those bounds before it enters downstream derivation.
+
 ## Warning semantics
 
 `SourceWarning` has a source name, stable code, severity, explanatory message,