docs

docs/architecture/website/DISPLAY_OBJECTS.md

@@ -25,7 +25,7 @@ They exist to avoid duplicating presentation logic across View Models.
 A Display Object MAY:
 
 - format values (money, dates, durations)
-- handle localization and language-specific rules
+- handle localization only when localization inputs are deterministic (for example: mapping stable codes to stable labels)
 - map codes to labels
 - encapsulate UI display conventions
 - be reused across multiple View Models
@@ -67,6 +67,20 @@ In this repository, a Display Object MUST NOT:
 
 Reason: these are runtime-locale/timezone dependent and cause SSR/hydration mismatches.
 
+## Localization rule (strict)
+
+Localization MUST NOT depend on runtime locale APIs.
+
+Allowed approaches:
+
+- API returns the exact labels/strings for the current user context.
+- Website maps stable codes to stable labels using a deterministic table.
+
+Forbidden approaches:
+
+- any usage of `Intl.*`
+- any usage of `toLocale*`
+
 If a rule affects system correctness or persistence,
 it does not belong in a Display Object.