order-parse: document that both order lanes must send the RFC Message-ID

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/lib/order-parse.ts

@@ -526,7 +526,11 @@ export function parseOrderHTML(html: string, meta: MessageMeta): ParsedOrder {
   } else {
     // DoorDash carries no order id anywhere in the receipt, so the provider
     // message id is the only stable identity available. That is correct for
-    // ingestion idempotency: one receipt is one order.
+    // ingestion idempotency: one receipt is one order — PROVIDED every caller
+    // sends the same id for the same email. Two lanes post here (n8n Order
+    // Processor and ingestion-engine order_handoff); both must send the bare
+    // RFC Message-ID. n8n sent Gmail's own id until 2026-10-03 and one order
+    // became txns 7268 + 7269.
     if (!meta.messageId) {
       throw new OrderParseError("no order id in body and no messageId supplied");
     }