refactor(providers): centralize provider-specific parsing and invariants

- Introduce internal/providers/nws with shared timestamp parsing used by both
  NWS sources and normalizers
- Migrate NWS observation source + normalizer to use the shared provider helper
  for consistent RFC3339/RFC3339Nano handling
- Introduce internal/providers/openweather with a shared URL invariant helper
  enforcing units=metric
- Remove duplicated OpenWeather URL validation logic from the observation source
- Align provider layering: move provider “contract/quirk” logic out of
  normalizers and into internal/providers
- Update normalizer and standards documentation to clearly distinguish:
  provider helpers (internal/providers) vs canonical mapping logic
  (internal/normalizers)

This refactor reduces duplication between sources and normalizers, clarifies
layering boundaries, and establishes a scalable pattern for future forecast
and alert implementations.

internal/normalizers/nws/observation.go

@@ -10,6 +10,7 @@ import (
 	"gitea.maximumdirect.net/ejr/feedkit/event"
 	"gitea.maximumdirect.net/ejr/weatherfeeder/internal/model"
 	normcommon "gitea.maximumdirect.net/ejr/weatherfeeder/internal/normalizers/common"
+	nwscommon "gitea.maximumdirect.net/ejr/weatherfeeder/internal/providers/nws"
 	"gitea.maximumdirect.net/ejr/weatherfeeder/internal/standards"
 )
 
@@ -46,11 +47,11 @@ func buildObservation(parsed nwsObservationResponse) (model.WeatherObservation,
 	// Timestamp (RFC3339)
 	var ts time.Time
 	if s := strings.TrimSpace(parsed.Properties.Timestamp); s != "" {
-		t, err := time.Parse(time.RFC3339, s)
+		t, err := nwscommon.ParseTime(s)
 		if err != nil {
 			return model.WeatherObservation{}, time.Time{}, fmt.Errorf("invalid timestamp %q: %w", s, err)
 		}
-		ts = t
+		ts = t.UTC()
 	}
 
 	cloudLayers := make([]model.CloudLayer, 0, len(parsed.Properties.CloudLayers))