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/providers/openweather/url.go

@@ -0,0 +1,26 @@
+package openweather
+
+import (
+	"fmt"
+	"net/url"
+	"strings"
+)
+
+// RequireMetricUnits enforces weatherfeeder's OpenWeather invariant:
+// the request URL must include units=metric (otherwise temperatures/winds/pressure differ).
+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
+}