fix: issue #35 validation expected unrendered .lang translation variant routes; main-language lists now render main-language translations

specs/generation.allium

@@ -113,6 +113,17 @@ invariant MultiLanguageRoutes {
     -- Each language subtree gets its own feeds and archives
 }
 
+invariant LanguageVariantSelection {
+    -- Every language tree renders each post in its own language when a
+    -- matching variant exists, falling back to the post's original language.
+    -- This applies uniformly to single pages, list pages (home/pagination,
+    -- category/tag/date archives), and feeds:
+    --   Main tree: a post whose language differs from main_language renders
+    --   its main_language translation when one exists (canonical variant).
+    --   Additional-language trees: posts resolve to that language's
+    --   translation; do_not_translate posts are excluded from those trees.
+}
+
 invariant CanonicalBaseUrlConfigured {
     for generation in SiteGenerations:
         generation.base_url != ""