feat(products): serialize multi-variant + interleaved images (§6.5.1 row grammar)

backend/app/domains/products/serialize.py

@@ -71,7 +71,9 @@ def _drain(buf: io.StringIO) -> str:
 
 
 def _product_rows(product: CatalogProduct) -> list[dict[str, str]]:
-    """The product's CSV rows (§6.5.1 grammar). Task 1: first variant only."""
+    """The product's CSV rows (§6.5.1 grammar): product fields + option names on
+    the first row; one variant per row; images interleaved; image-only rows when
+    a product has more images than variants."""
     base = {"Handle": product.handle, "Title": product.title}
     for field, col in _PRODUCT_FIELD_TO_COL.items():
         if field in product.fields:
@@ -79,10 +81,25 @@ def _product_rows(product: CatalogProduct) -> list[dict[str, str]]:
     for slot, name in enumerate(product.option_names, start=1):
         if name:
             base[f"Option{slot} Name"] = name
-    variant = product.variants[0]
-    row = dict(base)
-    _write_variant(row, product, variant)
-    return [row]
+
+    count = max(len(product.variants), len(product.images))
+    rows: list[dict[str, str]] = []
+    for i in range(count):
+        # Row 0 carries the product-level fields; later rows carry only Handle.
+        row = dict(base) if i == 0 else {"Handle": product.handle}
+        if i < len(product.variants):
+            _write_variant(row, product, product.variants[i])
+        if i < len(product.images):
+            _write_image(row, product.images[i])
+        rows.append(row)
+    return rows
+
+
+def _write_image(row: dict[str, str], image) -> None:
+    row["Image Src"] = image.source_url
+    row["Image Position"] = str(image.position)
+    if image.alt_text is not None:
+        row["Image Alt Text"] = image.alt_text
 
 
 def _write_variant(row: dict[str, str], product: CatalogProduct, variant) -> None: