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/openweather/observation.go

@@ -5,12 +5,11 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
-	"net/url"
-	"strings"
 	"time"
 
 	"gitea.maximumdirect.net/ejr/feedkit/config"
 	"gitea.maximumdirect.net/ejr/feedkit/event"
+	owcommon "gitea.maximumdirect.net/ejr/weatherfeeder/internal/providers/openweather"
 	"gitea.maximumdirect.net/ejr/weatherfeeder/internal/sources/common"
 	"gitea.maximumdirect.net/ejr/weatherfeeder/internal/standards"
 )
@@ -27,7 +26,7 @@ func NewObservationSource(cfg config.SourceConfig) (*ObservationSource, error) {
 		return nil, err
 	}
 
-	if err := requireMetricUnits(hs.URL); err != nil {
+	if err := owcommon.RequireMetricUnits(hs.URL); err != nil {
 		return nil, fmt.Errorf("%s %q: %w", hs.Driver, hs.Name, err)
 	}
 
@@ -39,7 +38,7 @@ func (s *ObservationSource) Name() string { return s.http.Name }
 func (s *ObservationSource) Kind() event.Kind { return event.Kind("observation") }
 
 func (s *ObservationSource) Poll(ctx context.Context) ([]event.Event, error) {
-	if err := requireMetricUnits(s.http.URL); err != nil {
+	if err := owcommon.RequireMetricUnits(s.http.URL); err != nil {
 		return nil, fmt.Errorf("%s %q: %w", s.http.Driver, s.http.Name, err)
 	}
 
@@ -93,20 +92,3 @@ func (s *ObservationSource) fetchRaw(ctx context.Context) (json.RawMessage, open
 
 	return raw, meta, nil
 }
-
-func requireMetricUnits(rawURL string) error {
-	u, err := url.Parse(strings.TrimSpace(rawURL))
-	if err != nil {
-		return fmt.Errorf("invalid url %q: %w", rawURL, err)
-	}
-
-	units := strings.ToLower(strings.TrimSpace(u.Query().Get("units")))
-	if units != "metric" {
-		if units == "" {
-			units = "(missing; defaults to standard)"
-		}
-		return fmt.Errorf("url must include units=metric (got units=%s)", units)
-	}
-
-	return nil
-}