refactor(normalizers): deduplicate synthetic station ID generation

- Add common SynthStationID helpers for coordinate-based providers
- Use shared helper for Open-Meteo and OpenWeather station ID synthesis
- Require both lat/lon when generating synthetic IDs to avoid misleading defaults
- Remove unused Open-Meteo normalizer wrapper code

This reduces cross-provider duplication while keeping provider-specific
mapping logic explicit and readable.

internal/normalizers/common/id.go

@@ -0,0 +1,23 @@
+// FILE: internal/normalizers/common/id.go
+package common
+
+import "fmt"
+
+// SynthStationID formats a stable synthetic station identifier for providers that are
+// coordinate-based rather than station-based.
+//
+// Example output:
+//
+//	OPENMETEO(38.62700,-90.19940)
+func SynthStationID(prefix string, lat, lon float64) string {
+	return fmt.Sprintf("%s(%.5f,%.5f)", prefix, lat, lon)
+}
+
+// SynthStationIDPtr is the pointer-friendly variant.
+// If either coordinate is missing, it returns "" (unknown).
+func SynthStationIDPtr(prefix string, lat, lon *float64) string {
+	if lat == nil || lon == nil {
+		return ""
+	}
+	return SynthStationID(prefix, *lat, *lon)
+}

internal/normalizers/common/round.go

@@ -56,14 +56,11 @@ func roundValue(v reflect.Value, decimals int) reflect.Value {
 			out.Set(elem)
 			return out
 		}
-		if elem.IsValid() && elem.Type().AssignableTo(v.Type()) {
-			out.Set(elem)
-			return out
-		}
 		if elem.IsValid() && elem.Type().ConvertibleTo(v.Type()) {
 			out.Set(elem.Convert(v.Type()))
 			return out
 		}
+
 		// If we can't sensibly re-wrap, just keep the original.
 		return v
 	}