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/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
 	}