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/sources/nws/observation.go

@@ -9,6 +9,7 @@ import (
 
 	"gitea.maximumdirect.net/ejr/feedkit/config"
 	"gitea.maximumdirect.net/ejr/feedkit/event"
+	nwscommon "gitea.maximumdirect.net/ejr/weatherfeeder/internal/providers/nws"
 	"gitea.maximumdirect.net/ejr/weatherfeeder/internal/sources/common"
 	"gitea.maximumdirect.net/ejr/weatherfeeder/internal/standards"
 )
@@ -85,8 +86,8 @@ func (s *ObservationSource) fetchRaw(ctx context.Context) (json.RawMessage, obse
 
 	tsStr := strings.TrimSpace(meta.Properties.Timestamp)
 	if tsStr != "" {
-		if t, err := time.Parse(time.RFC3339, tsStr); err == nil {
-			meta.ParsedTimestamp = t
+		if t, err := nwscommon.ParseTime(tsStr); err == nil {
+			meta.ParsedTimestamp = t.UTC()
 		}
 	}