normalizers/openweather: extract shared helpers into common.go

Refactor OpenWeather normalizers to improve structure and reuse by moving
provider-specific helper functions out of observation.go and into a new
common.go.

This keeps observation.go focused on schema matching and domain mapping,
preserves the “one normalizer per file” convention, and establishes a clear
home for helpers that will be shared by future OpenWeather forecast and alert
normalizers.

No functional behavior changes; this is a pure internal refactor.

internal/normalizers/openweather/common.go

@@ -0,0 +1,71 @@
+// FILE: ./internal/normalizers/openweather/common.go
+package openweather
+
+import (
+	"fmt"
+	"strings"
+)
+
+// This file holds provider-specific helpers that are shared across multiple
+// OpenWeather normalizers (observations today; forecasts/alerts later).
+// Keeping these out of observation.go helps preserve the "one normalizer per file"
+// convention while avoiding duplication.
+
+// primaryCondition returns the "primary" weather condition from OpenWeather's
+// weather array. Per OpenWeather conventions, element [0] is treated as primary.
+func primaryCondition(list []owmWeather) (id int, desc string, icon string) {
+	if len(list) == 0 {
+		return 0, "", ""
+	}
+	w := list[0]
+	return w.ID, strings.TrimSpace(w.Description), strings.TrimSpace(w.Icon)
+}
+
+// inferIsDay determines day/night using the best available upstream signals.
+//
+// Priority:
+//  1. The OpenWeather icon suffix ("d" / "n") when present.
+//  2. Sunrise/sunset bounds (unix seconds), if provided.
+//  3. Unknown (nil) when no reliable signal is present.
+func inferIsDay(icon string, dt, sunrise, sunset int64) *bool {
+	// Prefer icon suffix.
+	icon = strings.TrimSpace(icon)
+	if icon != "" {
+		last := icon[len(icon)-1]
+		switch last {
+		case 'd':
+			v := true
+			return &v
+		case 'n':
+			v := false
+			return &v
+		}
+	}
+
+	// Fall back to sunrise/sunset bounds if provided.
+	if dt > 0 && sunrise > 0 && sunset > 0 {
+		v := dt >= sunrise && dt < sunset
+		return &v
+	}
+
+	return nil
+}
+
+// openWeatherIconURL builds the standard OpenWeather icon URL for the given icon code.
+func openWeatherIconURL(icon string) string {
+	icon = strings.TrimSpace(icon)
+	if icon == "" {
+		return ""
+	}
+	return fmt.Sprintf("https://openweathermap.org/img/wn/%s@2x.png", icon)
+}
+
+// openWeatherStationID returns a stable station identifier for the given response.
+// Prefer the OpenWeather city ID when present; otherwise, fall back to coordinates.
+func openWeatherStationID(parsed owmResponse) string {
+	if parsed.ID != 0 {
+		return fmt.Sprintf("OPENWEATHER(%d)", parsed.ID)
+	}
+	// Fallback: synthesize from coordinates.
+	return fmt.Sprintf("OPENWEATHER(%.5f,%.5f)", parsed.Coord.Lat, parsed.Coord.Lon)
+}