docs: addressing contract — per-language slugs and the root KEY namespace

DESIGN.md

@@ -122,6 +122,23 @@ justified; foundational frameworks/libraries chosen deliberately.
 
 ParVagues is the flagship precedent and keeps its own SPA shell.
 
+## Addressing — per-language slugs + the root KEY namespace
+
+> Owner, verbatim: *"i want a per-lang slug often. so default to /slug/lang is ok, but lets
+> have /slug-per-lang work directly too. thus me.nech.pl/KEY can be {project, slug-per-lang,
+> slug-of-any-lang}"*
+
+- **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.
+- **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,
+  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.
+
 ## /atlas — the living-field side app
 
 Name ratified: *"**/atlas** c'est cool c'est stellaire fr et en lets go :)"*