fix(products): cleared Variant Position resolves to file order, not NULL

A blank Variant Position cell previewed position->null and aborted the
confirm transaction (variant.position is NOT NULL). Resolve the clear to
the variant's file order at diff time so preview and apply stay in
lockstep; carry file_order on VariantPlan instead of an identity-keyed
map; release the read snapshot before PreviewStale/NothingToApply.

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

backend/tests/test_products_diff.py

@@ -113,6 +113,24 @@ def test_changed_variant_position_reports_honest_before():
     assert {"field": "position", "before": 1, "after": 2} in ventry["changes"]
 
 
+def test_blank_position_cell_resolves_to_file_order_unchanged():
+    # A present-but-empty Variant Position cell resets to file order (the spec's
+    # "defaults to file order"), never to NULL — here file order matches the
+    # catalog position, so nothing changes.
+    diff = compute_diff(_catalog_mug(), _canon(POSITION_HEADER, MUG_ROW + ","))
+    assert diff.records[0]["kind"] == "unchanged"
+
+
+def test_blank_position_cell_updates_to_file_order():
+    catalog = _catalog_mug()
+    catalog["moon-mug"].variants[0].position = 2
+    diff = compute_diff(catalog, _canon(POSITION_HEADER, MUG_ROW + ","))
+    [rec] = diff.records
+    assert rec["kind"] == "update"
+    [ventry] = rec["detail"]["variants"]
+    assert {"field": "position", "before": 2, "after": 1} in ventry["changes"]
+
+
 def test_error_product_classifies_error():
     diff = compute_diff({}, _canon("Handle,Title,Variant Price", "mug,Mug,nope"))
     [rec] = diff.records