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>

backend/app/domains/products/serialize.py

@@ -109,6 +109,11 @@ def _write_variant(row: dict[str, str], product: CatalogProduct, variant) -> Non
         # (a no-option product's single variant carries all-NULL options).
         if product.option_names[slot - 1] and value is not None:
             row[f"Option{slot} Value"] = value
+    # position is a CatalogVariant attribute, not a fields{} entry — emit it
+    # explicitly. An empty Variant Position cell re-imports as "reset to file
+    # order", so a non-sequential stored position would round-trip to an update
+    # (INV-12). (Like _write_image, which emits its position attribute.)
+    row["Variant Position"] = str(variant.position)
     for field, col in _VARIANT_FIELD_TO_COL.items():
         if field in variant.fields:
             row[col] = _cell(variant.fields[field])

backend/tests/test_products_serialize.py

@@ -136,12 +136,15 @@ def _gen_catalog(seed: int) -> dict:
         if has_opts:
             sizes = rng.sample(["S", "M", "L", "XL"], rng.randint(1, 4))
             for vi, size in enumerate(sizes, start=1):
+                # Non-sequential positions (×10) so a serializer that drops the
+                # stored position and lets re-import default to file order is
+                # caught — a merchant can import explicit positions like 10/20.
                 variants.append(CatalogVariant(
-                    id=pid * 100 + vi, options=(size, "Indigo", None), position=vi,
+                    id=pid * 100 + vi, options=(size, "Indigo", None), position=vi * 10,
                     fields={"sku": f"SKU-{pid}-{vi}", "variant_image": None}))
         else:
             variants.append(CatalogVariant(
-                id=pid * 100 + 1, options=(None, None, None), position=1,
+                id=pid * 100 + 1, options=(None, None, None), position=rng.choice([1, 5, 9]),
                 fields={"sku": f"SKU-{pid}", "variant_image": None}))
         images = []
         for ii in range(rng.randint(0, 3)):
@@ -194,3 +197,28 @@ def test_decimal_and_int_variant_fields_roundtrip():
     result = _roundtrip_diff(catalog)
     assert result.summary["unchanged"] == 1, result.records
     assert result.summary["updates"] == 0
+
+
+def test_nonsequential_variant_position_roundtrips():
+    """A stored variant position that isn't its file order must survive export —
+    else re-import reads the empty cell as 'reset to file order' and the round-trip
+    spuriously updates (INV-12 regression: position is a CatalogVariant attribute,
+    not a fields{} entry, so the serializer must emit it explicitly)."""
+    catalog = {
+        "tee": CatalogProduct(
+            id=1, handle="tee", title="Tee", option_names=("Size", None, None),
+            fields={"title": "Tee", "description_html": None, "vendor": None,
+                    "product_type": "standalone", "google_product_category": None,
+                    "tags": [], "status": "active", "published": True},
+            variants=[
+                CatalogVariant(id=1, options=("S", None, None), position=10,
+                               fields={"sku": "T-S", "variant_image": None}),
+                CatalogVariant(id=2, options=("M", None, None), position=20,
+                               fields={"sku": "T-M", "variant_image": None}),
+            ],
+            images=[],
+        )
+    }
+    result = _roundtrip_diff(catalog)
+    assert result.summary["unchanged"] == 1, result.records
+    assert result.summary["updates"] == 0

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
+// Products page (SD-0002 §5.2) — the catalog's home: where imports start, exports
-// lives. SLICE-5: export is disabled (SLICE-6 ships it); the browsable list is #14's.
+// 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">