docs: root form is canonical for posts

DESIGN.md

@@ -138,6 +138,9 @@ ParVagues is the flagship precedent and keeps its own SPA shell.
   admin, images…) fails the build loudly instead of silently shadowing it.
 - Canonical URLs are declared once per post (canonical link tags); alternate-language and
   root-key URLs serve the same content and point at the canonical.
+  **Ratified: the root form is canonical** — `me.nech.pl/<slug>` is the real address;
+  `/blog/<slug>` serves the same page pointing canonical at the root. RSS, share links,
+  and metadata all use the root form.
 
 ## /atlas — the living-field side app