fix(products): conservative dialect detection — canonical-distinctive columns veto Shopify (SLICE-8 review)

Addresses adversarial-review findings: a canonical file carrying a stray
Shopify-signature name (e.g. SEO Title) no longer misdetects as Shopify and
silently drops canonical Type / corrupts the weight unit (kg->g). A
canonical-distinctive column (Description/Variant Cost/Variant Weight/Google
Product Category/Variant Volume/Tax ID/Position) now vetoes detection, so
detection leans conservative (under-detection warns; over-detection corrupts).
Also closes the dual-named-column shadow (Body (HTML)+Description) and the
stale-weight-unit-on-clear case. Tests cover all three.

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

backend/app/domains/products/codec.py

@@ -73,9 +73,13 @@ def parse_csv(data: bytes) -> ParsedFile:
                 c: (raw[col_index[c]].strip() if col_index[c] < len(raw) else "")
                 for c in col_index
             }
-            # Shopify grams carry an implicit unit; canonical needs it explicit (§6.5.1).
-            if dialect == "shopify" and cells.get("Variant Weight"):
-                cells["Variant Weight Unit"] = "g"
+            # Shopify grams carry an implicit unit; canonical needs it explicit. In a
+            # Shopify file "Variant Weight" can only come from the Variant Grams rename
+            # (a canonical-named Variant Weight would have vetoed Shopify detection), so
+            # weight and unit move together: a value -> "g"; a cleared grams (present-but-
+            # empty) clears the unit too, never leaving a stale unit (§6.5.1).
+            if dialect == "shopify" and "Variant Weight" in cells:
+                cells["Variant Weight Unit"] = "g" if cells["Variant Weight"] else ""
             rows.append(Row(line_number=reader.line_num, cells=cells))
     except csv.Error:
         raise FileRejected("not_csv", "This file isn't readable as CSV.") from None