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-09-pure-logic.md

@@ -59,7 +59,7 @@ no spec despite "domain and pure logic must have a spec" (CLAUDE.md §5).
       (`draft-sync.spec.ts`, `submit-change-request.spec.ts`).
 - [x] `map3` removed (found in `shared/application/remote-data.ts`, not
       `shared/kernel/fp.ts` as the WP text guessed — updated the three docs that
-      mentioned it: CLAUDE.md, `docs/ARCHITECTURE.md`, `remote-data.mdx`). The
+      mentioned it: CLAUDE.md, `docs/reference/architecture/ARCHITECTURE.md`, `remote-data.mdx`). The
       `variant` input on `confirmation.component.ts` no longer exists — already
       cleaned up before this WP ran; nothing to do.
 - [x] CLAUDE.md rule added (`Conventions` — DatePipe in templates, `formatDatumNl` in