Validate hourly forecast time bounds

docs/internal/collect.md

@@ -20,6 +20,9 @@ Fetch failures retain Weather API endpoint context but do not project upstream
 response bodies into application-facing errors. Oversized response bodies fail
 collection before source decoding.
 
+The required hourly product must contain one or more periods with usable time
+bounds. An invalid hourly product fails collection before derivation begins.
+
 ## Application Composition
 
 `internal/app` owns the narrow `Collector` interface used by workflow tests;