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/app/domains/products/diff.py

@@ -9,7 +9,8 @@ Deliberately DB-free: the catalog snapshot dataclasses are defined here and the
 repo layer builds them. Only fields present in the file's canonical fields{}
 participate in a comparison — an absent column is untouched, never a change
 (§6.5.1); a present-but-empty cell resolves to the field's CLEAR_DEFAULTS entry.
-Catalog variants/images absent from the file are likewise untouched (INV-10).
+Catalog variants/images absent from the file are likewise untouched (INV-10);
+file variants match catalog variants by their option-value combination (INV-13).
 """
 from __future__ import annotations
 
@@ -89,7 +90,7 @@ def compute_diff(catalog: dict[str, CatalogProduct], products: list[CanonicalPro
         )
         summary[_SUMMARY_KEY[kind]] += 1
     fingerprint = hashlib.sha256(
-        json.dumps(records, sort_keys=True, separators=(",", ":"), default=str).encode()
+        json.dumps(records, sort_keys=True, separators=(",", ":")).encode()
     ).hexdigest()
     return DiffResult(records=records, summary=summary, fingerprint=fingerprint)
 
@@ -140,11 +141,13 @@ def _update_detail(product: CanonicalProduct, current: CatalogProduct) -> dict:
                 {"options": list(variant.options), "kind": "add", "set": _resolved_variant_fields(variant)}
             )
             continue
-        variant_changes = [
-            _change(f, match.fields.get(f), resolved)
-            for f, resolved in _resolved_fields(variant.fields).items()
-            if resolved != match.fields.get(f)
-        ]
+        variant_changes = []
+        for f, resolved in _resolved_fields(variant.fields).items():
+            # position lives on the catalog variant as an attribute, not in
+            # fields{} — compare it explicitly (as images do for theirs).
+            before = match.position if f == "position" else match.fields.get(f)
+            if resolved != before:
+                variant_changes.append(_change(f, before, resolved))
         if variant_changes:
             variant_entries.append(
                 {"options": list(variant.options), "kind": "update", "changes": variant_changes}