docs(products): DOC-4 dialect-adapter notes (SLICE-8)

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

docs/products-domain.md

@@ -60,6 +60,42 @@ Option names live both in `CanonicalProduct.option_names` (the values) and in
 `fields{}` (the file-presence marker the diff needs for the absent-vs-clear
 distinction).
 
+## Dialects (INV-17, SLICE-8)
+
+A file is normalized to canonical names **at the codec boundary**, so every stage
+downstream of `parse_csv` (validate → diff → apply) is dialect-agnostic — it only
+ever sees canonical cells. Two dialects exist: `canonical` and `shopify`.
+
+`dialect_shopify.py` is the Shopify adapter and the source of truth for the
+mapping (mirrored by `tests/test_products_dialect_shopify.py` and
+`tests/fixtures/shopify-export.csv`):
+
+- **Detection** — `is_shopify_header()` returns `shopify` iff the header carries a
+  Shopify-only *signature* column (`Body (HTML)`, `Variant Grams`, `Cost per item`,
+  `Gift Card`, a `Google Shopping / …` column, …). Everything else is `canonical`.
+  Detection is by signature, not an exact header set, so extra market/region columns
+  don't defeat it; an ambiguous header (only columns common to both dialects) falls
+  through to `canonical`, which maps those shared columns identically — so it never
+  misparses (§7.4).
+- **Mapping** — `map_shopify_header(header)` returns `(mapped, not_imported)`:
+  `mapped[i]` is the canonical name for column `i` (or `None` when it has no
+  canonical home), and `not_imported` is the warning list surfaced as the draft's
+  `unknown_columns`. The rule per column, in order: in `SHOPIFY_RENAME` →
+  canonical name; in `SHOPIFY_DROP` (`Type`, `Variant Weight Unit`) → not imported;
+  in `KNOWN_COLUMNS` → pass through; else → not imported (this last branch absorbs
+  the unbounded `… / <Market>` price columns without enumerating them).
+- **The two name-collision overrides** are why `SHOPIFY_DROP` exists: Shopify's
+  `Type` is a free-text category, but canonical `Type` is *structural*
+  (`standalone`/kits), so it is warned rather than folded in; and Shopify's
+  `Variant Weight Unit` is dropped because the `Variant Grams` → `Variant Weight`
+  rename is the weight source.
+- **The one value transform** lives in `parse_csv`: when a Shopify row has a mapped
+  `Variant Weight` (from `Variant Grams`), the codec synthesizes
+  `Variant Weight Unit = "g"`, since Shopify grams are unitless integers.
+
+`detect_dialect()` in `codec.py` is the INV-17 seam; a new dialect is a new adapter
+module plus a branch there, with no change to validate/diff/apply.
+
 ## Error granularity
 
 `validate.py` never raises on a row problem: every violation is recorded as a