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/bdd.mdx

@@ -0,0 +1,75 @@
+import { Meta } from '@storybook/addon-docs/blocks';
+
+<Meta title="Foundations/BDD" />
+
+# Behaviour-driven tests
+
+Tests here read as **specifications of behaviour**, not checks of implementation. A test
+says what the system *does* — in the domain's own words — so a failing test names a broken
+behaviour, and the suite doubles as living documentation. This is the BDD half of the
+[Testing strategy](?path=/docs/foundations-testing-strategy--docs) (which owns *what to
+test, by layer*); BDD owns *how each test is phrased and scoped*.
+
+## Three rules
+
+### 1. `describe` = the subject, `it` = one observable behaviour
+
+The `describe()` block names the unit under test; each `it()` states a single behaviour in
+**declarative present tense** — the implicit subject is "it". No `should`, no
+Given/When/Then ceremony: present-tense declaration already reads as a spec.
+
+```ts
+describe('parsePostcode', () => {
+  it('normalises to "1234 AB" (uppercase, single space, trimmed)', () => { … });
+  it('rejects malformed input', () => { … });
+});
+```
+
+Read top-to-bottom it *is* the spec: "parsePostcode — normalises to 1234 AB; rejects
+malformed input."
+
+### 2. One behaviour per test
+
+A test asserts **one behaviour**, not one `expect()`. Several assertions that pin down the
+*same* behaviour belong together; assertions about *different* behaviours belong apart.
+
+| Keep together (one behaviour) | Split apart (separate behaviours) |
+| --- | --- |
+| A `Result`'s `.ok` then its `.value` | The `ok` branch **and** the `err` branch of a transition |
+| A whole-object `toEqual` | An invalid-input case **and** a valid-input case |
+| A loop asserting one rule over many inputs | Two independent state transitions |
+| A truth-table (`draft` → true, `approver` → false) of one rule | An authorization check **and** a rendering check |
+
+A title that needs `/`, `;`, "then" or "and" to join two behaviours is the smell — split it,
+and each half gets its own present-tense name.
+
+### 3. Speak the ubiquitous language (the DDD tie-in)
+
+Test names use the **domain vocabulary**, not technical jargon — the same words as the
+[bounded contexts](?path=/docs/foundations-domain-driven-design--docs): a *behandelaar*
+drafts, a *beoordelaar* approves, a *herregistratie* is *ingediend*. The test name is
+readable by someone who knows the domain but not the code.
+
+```ts
+it('drafter cannot approve or reject even when submitted', …);
+it('confirmed dutch proficiency requires taalvaardigheid proof', …);
+```
+
+## How it fits TDD & DDD
+
+- **TDD** — the loop is red → green → refactor: write the behaviour as a failing `it`, make
+  it pass, then clean up. Because tests describe behaviour (not internals), a refactor that
+  preserves behaviour keeps them green. Pure domain logic is tested directly — no `TestBed`
+  (see [Testing strategy](?path=/docs/foundations-testing-strategy--docs)).
+- **DDD** — behaviour is expressed in the ubiquitous language, so the spec and the code
+  share one vocabulary. Domain rules (reducers, value-object parsers, policies) are the
+  richest specs; the wire boundary is tested as "rejects malformed input", the UI as
+  Storybook stories.
+
+## Where to look
+
+Canonical behaviour specs in the repo: `registratie/domain/value-objects/postcode.spec.ts`
+(parser behaviour), `registratie/domain/registratie-wizard.machine.spec.ts` (one transition
+per test), and backend `AuthzTests.cs` (rule truth-tables). The
+[Testing strategy](?path=/docs/foundations-testing-strategy--docs) page maps which layer
+gets which kind of test.