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>

src/docs/overview.mdx

@@ -0,0 +1,35 @@
+import { Meta } from '@storybook/addon-docs/blocks';
+
+<Meta title="Foundations/Overview" />
+
+# Foundations
+
+The **why** behind this codebase, as a short curriculum. Each page is a condensed,
+cross-linked take on one idea; the long-form source lives in `docs/reference/`
+(see the repo's `docs/README.md`). Read them in roughly this order.
+
+## Architecture & domain
+
+- [Domain-driven design](?path=/docs/foundations-domain-driven-design--docs) — bounded
+  contexts + layers, dependencies pointing inward (the folder structure *is* the architecture).
+- [Atomic design](?path=/docs/foundations-atomic-design--docs) — Atoms → Molecules →
+  Organisms → Templates; a new page is composition, not new building blocks.
+
+## Functional core
+
+- [FP in the UI](?path=/docs/foundations-fp-in-the-ui--docs) — the three functional tools behind the view.
+- [State machines (TEA)](?path=/docs/foundations-state-machines-tea--docs) — every form/wizard as Model → Msg → pure `reduce`.
+- [RemoteData & Async](?path=/docs/foundations-remotedata-async--docs) — the four async states as one value.
+- [Parse, don't validate](?path=/docs/foundations-parse-dont-validate--docs) — narrow untrusted `unknown` at the boundary into domain types.
+
+## Design system
+
+- [Design tokens](?path=/docs/foundations-design-tokens--docs) — semantic `--rhc-*` tokens; no hand-written colours.
+- [CIBG gap register](?path=/docs/foundations-cibg-gap-register--docs) — where we diverge from the CIBG Huisstijl (ADR-0003).
+
+## Quality & process
+
+- [Accessibility](?path=/docs/foundations-accessibility--docs) — four layered a11y tools, each catching a different bug class.
+- [Testing strategy](?path=/docs/foundations-testing-strategy--docs) — what to test, by layer grain.
+- [BDD](?path=/docs/foundations-bdd--docs) — how each test is phrased and scoped: one behaviour, in the domain's language.
+- [Internationalization](?path=/docs/foundations-internationalization--docs) — `$localize` for every user-visible string; the locale seam.