normalizers: added a structure for normalizers; refactoring sources -> sources+normalizers is still todo.

internal/normalizers/openweather/register.go

@@ -0,0 +1,19 @@
+package openweather
+
+import (
+	fknormalize "gitea.maximumdirect.net/ejr/feedkit/normalize"
+)
+
+// Register registers OpenWeather normalizers into the provided registry.
+//
+// This is intentionally empty as a stub. As normalizers are implemented,
+// register them here, e.g.:
+//
+//	reg.Register(ObservationNormalizer{})
+func Register(reg *fknormalize.Registry) {
+	if reg == nil {
+		return
+	}
+
+	// TODO: register OpenWeather normalizers here.
+}