Add day/night estimation for NWS observations based on solar elevation and update observation model.

internal/normalizers/nws/observation.go

@@ -76,9 +76,13 @@ func buildObservation(parsed nwsObservationResponse) (model.WeatherObservation,
 	// Determine canonical WMO condition code.
 	wmo := mapNWSToWMO(providerDesc, cloudLayers, phenomena)
 
+	var isDay *bool
+	if lat, lon := observationLatLon(parsed.Geometry.Coordinates); lat != nil && lon != nil {
+		isDay = isDayFromLatLonTime(*lat, *lon, ts)
+	}
+
 	// Canonical condition text comes from our WMO table.
-	// NWS observation responses typically do not include a day/night flag -> nil.
-	canonicalText := standards.WMOText(wmo, nil)
+	canonicalText := standards.WMOText(wmo, isDay)
 
 	// Apparent temperature: prefer wind chill when both are supplied.
 	var apparentC *float64
@@ -95,7 +99,7 @@ func buildObservation(parsed nwsObservationResponse) (model.WeatherObservation,
 
 		ConditionCode: wmo,
 		ConditionText: canonicalText,
-		IsDay:         nil,
+		IsDay:         isDay,
 
 		ProviderRawDescription: providerDesc,