Refactor normalizers: dedupe JSON decode + event finalize

Add shared normalizer helpers to centralize payload extraction, JSON decoding,
and event finalization/validation.

Refactor NWS, Open-Meteo, and OpenWeather observation normalizers to use the
shared spine, removing repeated boilerplate while preserving provider-specific
mapping logic.

internal/normalizers/common/finalize.go

@@ -0,0 +1,32 @@
+// FILE: ./internal/normalizers/common/finalize.go
+package common
+
+import (
+	"time"
+
+	"gitea.maximumdirect.net/ejr/feedkit/event"
+)
+
+// Finalize builds the output event envelope by copying the input and applying the
+// canonical schema/payload, plus (optionally) EffectiveAt.
+//
+// Important behavior:
+//   - ID/Kind/Source/EmittedAt are preserved by copying the input event.
+//   - EffectiveAt is only overwritten when effectiveAt is non-zero.
+//     If effectiveAt is zero, any existing in.EffectiveAt is preserved.
+func Finalize(in event.Event, outSchema string, outPayload any, effectiveAt time.Time) (*event.Event, error) {
+	out := in
+	out.Schema = outSchema
+	out.Payload = outPayload
+
+	if !effectiveAt.IsZero() {
+		t := effectiveAt.UTC()
+		out.EffectiveAt = &t
+	}
+
+	if err := out.Validate(); err != nil {
+		return nil, err
+	}
+
+	return &out, nil
+}