docs: reorganize into project/ + reference/, expand Storybook Foundations

Move working docs (backlog, prd, roadmap) under docs/project/ and durable
docs (architecture ADRs, guides, audits) under docs/reference/; add a
docs/README.md index. Update every path reference in code comments, CLAUDE.md,
READMEs, and the new-ssp skill. Expand the Storybook Foundations curriculum
(Overview, BDD, i18n; rename Layers→Domain-Driven Design) and reorder the sidebar.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/project/backlog/WP-04-ui-not-infrastructure.md

@@ -11,7 +11,7 @@ Phase 0 — a pure move of wiring, no behavior change.
 
 ## Read first
 
-- `CLAUDE.md` §1, `docs/ARCHITECTURE.md`
+- `CLAUDE.md` §1, `docs/reference/architecture/ARCHITECTURE.md`
 - `eslint.config.mjs`
 - `src/app/registratie/ui/registratie-wizard/registratie-wizard.component.ts` (lines
   ~18-19: `BrpAdapter`/`parseBrpAddress`, `DuoAdapter`/`parseDuoLookup`)
@@ -38,7 +38,7 @@ Phase 0 — a pure move of wiring, no behavior change.
   lookups move behind a registratie `application/` facade (command or store methods)
 - `src/app/herregistratie/ui/intake-wizard/intake-wizard.component.ts` — policy resource
   moves behind **new** `src/app/herregistratie/application/` (folder doesn't exist yet)
-- `docs/ARCHITECTURE.md` — fix "three contexts, four layers" → six contexts
+- `docs/reference/architecture/ARCHITECTURE.md` — fix "three contexts, four layers" → six contexts
   (shared, auth, registratie, herregistratie, brief, showcase), five layers (+ contracts);
   add the showcase sanction
 - `CLAUDE.md` §1 — add `brief` to the context list; note the showcase sanction