Fix API Error before M5

docs/architecture.md

@@ -490,7 +490,25 @@ sent an English shopper following "load more" to the Vietnamese listing — invi
 default locale, which is exactly why it survived.
 
 So: **exercise a flow on data you just created, not only on seeded data, perform each action
-twice, and do it in a non-default locale.** Seed data has been through every code path already; new data has been through none. The
+twice, and do it in a non-default locale.**
+
+The sweep before M5 added one more, and it is the sharpest of the set. Two of the worst defects
+in the catalog write path were not logic errors at all — they were **library behaviour that had
+drifted out from under the intent written in the comments**:
+
+- `z.record(z.enum(LOCALES), …)` is exhaustive in Zod 4. The helper was named
+  `requiredTranslations`, its comment said "at least the default locale", and its `.refine`
+  checked exactly that — and was unreachable. Every single-language product was rejected.
+- `.partial()` makes a field optional but keeps its `.default([])`. So an omitted
+  `genderTargets` arrived as `[]`, and `[]` is truthy. Renaming a product cleared its targeting,
+  dropped every collection link and deleted all of its attributes.
+
+Both typechecked. Both read correctly. Neither had a test, because the behaviour they depended on
+was assumed rather than asserted. `packages/validation/src/catalog-admin.spec.ts` now pins both.
+
+So: **when a schema encodes an intent, assert the intent — not the shape.** "Absent means leave
+alone" and "one language is enough" are claims about behaviour, and a type signature cannot make
+either of them true. Seed data has been through every code path already; new data has been through none. The
 inventory case is pinned in `apps/api/src/modules/inventory/inventory-list.spec.ts`.
 
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