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

backend/app/domains/products/dialect_shopify.py

@@ -38,12 +38,36 @@ _SIGNATURE: frozenset[str] = frozenset(
     }
 )
 
+# Canonical-distinctive columns — names Shopify renames away (so a real Shopify export
+# never carries them) plus canonical-only columns Shopify has no equivalent for. Their
+# presence proves the file is canonical and VETOES Shopify detection: detection must
+# lean conservative, because under-detection is safe (Shopify-named columns are warned
+# as not-imported) while over-detection corrupts (canonical Type / weight-unit are
+# dropped). This closes the misdetection hazard — a canonical file that merely contains
+# a stray signature-named column (e.g. `SEO Title`) is never misread as Shopify (§7.4).
+_CANONICAL_SIGNATURE: frozenset[str] = frozenset(
+    {
+        "Description",
+        "Variant Cost",
+        "Variant Weight",
+        "Google Product Category",
+        "Variant Volume",
+        "Variant Volume Unit",
+        "Variant Tax ID 1",
+        "Variant Tax ID 2",
+        "Variant Position",
+    }
+)
+
 
 def is_shopify_header(header: list[str]) -> bool:
-    """True iff the header carries a Shopify-only signature column. Otherwise the
-    file is treated as canonical — shared columns map identically, so an ambiguous
-    header never misparses (§7.4)."""
+    """True iff the header carries a Shopify-only signature column AND no
+    canonical-distinctive column. A canonical signal vetoes Shopify detection so an
+    ambiguous or hybrid header is treated as canonical — where Shopify-named columns
+    are warned as not-imported rather than silently remapped (never misparses, §7.4)."""
     cols = {h.strip() for h in header}
+    if cols & _CANONICAL_SIGNATURE:
+        return False
     if cols & _SIGNATURE:
         return True
     return any(c.startswith("Google Shopping / ") for c in cols)