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;

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,