fix(products): honest position compare for matched variants in diff

A Variant Position column previously produced a spurious update with a
false before:None (CatalogVariant keeps position as an attribute, not in
fields{}). Compare against the attribute, like images already do.

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

backend/tests/test_products_diff.py

@@ -95,6 +95,24 @@ def test_new_option_combo_is_variant_add_existing_untouched():
     assert "add" in kinds
 
 
+POSITION_HEADER = HEADER + ",Variant Position"
+
+
+def test_matching_variant_position_column_classifies_unchanged():
+    # position is an attribute on CatalogVariant (not in fields{}); the compare
+    # must read it from there, not invent a before:None.
+    diff = compute_diff(_catalog_mug(), _canon(POSITION_HEADER, MUG_ROW + ",1"))
+    assert diff.records[0]["kind"] == "unchanged"
+
+
+def test_changed_variant_position_reports_honest_before():
+    diff = compute_diff(_catalog_mug(), _canon(POSITION_HEADER, MUG_ROW + ",2"))
+    [rec] = diff.records
+    assert rec["kind"] == "update"
+    [ventry] = rec["detail"]["variants"]
+    assert {"field": "position", "before": 1, "after": 2} in ventry["changes"]
+
+
 def test_error_product_classifies_error():
     diff = compute_diff({}, _canon("Handle,Title,Variant Price", "mug,Mug,nope"))
     [rec] = diff.records