fix(products): serialize Variant Position explicitly — INV-12 round-trip bug

A stored variant position is a CatalogVariant attribute, not a fields{} entry,
so the serializer emitted an empty Variant Position cell — which re-imports as
'reset to file order'. A non-sequential stored position (a merchant can import
explicit positions) then round-tripped to a spurious update, violating INV-12.
Emit str(variant.position) explicitly (like _write_image). The property-test
generator now assigns non-sequential positions to lock the regression, plus a
targeted test. Also wires isExportEnabled into ProductsPage (was dead code).
Both caught by the final code review.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/screens/products/ProductsPage.tsx

@@ -1,5 +1,5 @@
-// Products page (SD-0002 §5.2) — the catalog's home: where imports start and history
-// lives. SLICE-5: export is disabled (SLICE-6 ships it); the browsable list is #14's.
+// Products page (SD-0002 §5.2) — the catalog's home: where imports start, exports
+// download, and history lives. SLICE-6 ships the export menu; the browsable list is #14's.
 import { useEffect, useState } from "react";
 import {
   dialectLabel,
@@ -11,6 +11,7 @@ import {
   type RunSummary,
 } from "../../productsApi";
 import { Banner } from "../../ui/kit";
+import { isExportEnabled } from "./exportMenu";
 
 const STATUS_LABELS: Record<string, string> = {
   applying: "Importing…",
@@ -56,7 +57,7 @@ export default function ProductsPage() {
     );
   }
 
-  const empty = summary.product_count === 0;
+  const empty = !isExportEnabled(summary.product_count);
   return (
     <div className="products">
       <header className="products__header">