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/nws/doc.go

@@ -0,0 +1,8 @@
+// Package nws contains provider-specific helper code for the National Weather Service
+// used by both sources and normalizers.
+//
+// Rules:
+//   - No network I/O here (sources fetch; normalizers transform).
+//   - Keep helpers deterministic and easy to unit test.
+//   - Prefer putting provider quirks/parsing here when sources + normalizers both need it.
+package nws

internal/providers/nws/time.go

@@ -0,0 +1,27 @@
+package nws
+
+import (
+	"fmt"
+	"strings"
+	"time"
+)
+
+// ParseTime parses NWS timestamps.
+//
+// NWS observation timestamps are typically RFC3339, sometimes with fractional seconds.
+// We accept RFC3339Nano first, then RFC3339.
+func ParseTime(s string) (time.Time, error) {
+	s = strings.TrimSpace(s)
+	if s == "" {
+		return time.Time{}, fmt.Errorf("empty time")
+	}
+
+	if t, err := time.Parse(time.RFC3339Nano, s); err == nil {
+		return t, nil
+	}
+	if t, err := time.Parse(time.RFC3339, s); err == nil {
+		return t, nil
+	}
+
+	return time.Time{}, fmt.Errorf("unsupported NWS timestamp format: %q", s)
+}

internal/providers/openweather/doc.go

@@ -0,0 +1,8 @@
+// Package openweather contains provider-specific helper code for OpenWeather used by
+// both sources and normalizers.
+//
+// Rules:
+//   - No network I/O here.
+//   - Keep helpers deterministic and easy to unit test.
+//   - Put provider invariants here (e.g., units=metric enforcement).
+package openweather

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
+}