docs: addressing seam — localized slugs resolve at the root form only

DESIGN.md

@@ -130,8 +130,10 @@ ParVagues is the flagship precedent and keeps its own SPA shell.
 
 - **Per-language slugs:** each language file may carry its own localized `slug` (frontmatter
   of `fr.md`/`en.md`); the folder name stays the canonical slug. Every declared slug
-  resolves directly: canonical → default language, localized slug → that language, in both
-  the `/blog/<slug>` and root-KEY forms.
+  resolves directly as a root key (canonical → default language, localized slug → that
+  language). The `/blog/<slug>` alias form follows the *canonical* slug only — localized
+  slugs live at the root, which is canonical anyway. *(Amended 2026-10-07: the original
+  "both forms" wording overpromised; implementation and intent agree on root-only.)*
 - **Root KEY namespace:** `me.nech.pl/<KEY>` resolves at build time to a project page or a
   post in any of its languages. Root keys are premium real estate with a build-time
   collision guard: a key colliding with a real route (blog, projects, atlas, parvagues,