docs(storybook): render MDX tables + add i18n & testing-strategy docs/skill

- fix: wire remark-gfm into addon-docs so GFM pipe tables in *.mdx render
  (previously raw text in cibg-gaps/layers/atomic-design docs)
- add src/docs/i18n.mdx (Foundations/Internationalization): the $localize
  locale seam + how to test languages without coupling to copy
- add src/docs/testing.mdx (Foundations/Testing strategy): per-layer spec
  matrix, house style, Storybook a11y gate, GREEN gate
- add .claude/skills/test-strategy skill

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

.claude/skills/test-strategy/SKILL.md

@@ -0,0 +1,67 @@
+---
+name: test-strategy
+description: Place tests the house way — Vitest specs co-located by layer (pure domain, no TestBed; parse* trust boundaries; thin UI via Storybook a11y). Use whenever adding a spec or deciding what to test.
+---
+
+# Test strategy (test where it's pure)
+
+Push logic down to where it's pure, test it there directly, keep the layers above thin.
+No `TestBed` for domain. Never assert on user-facing copy.
+
+## Rules
+
+- **`domain/` + any pure logic → required spec.** Reducers, combinators, `visibleSteps`,
+  policies, parsers. Import the function and call it — no Angular, no `TestBed`.
+- **Value-object parser → happy path + normalisation + each rejection.** Assert on the
+  `Result` discriminant (`.ok`) and the parsed value, **not** the error message.
+- **`infrastructure/` `parse*` (trust boundary) → required spec.** Accept a valid DTO;
+  reject `null`, `{}`, and malformed shapes. Name it `describe('… (trust boundary)')`.
+- **`application/` stores/commands → spec** the pure reduce + optimistic
+  begin→confirm/rollback + the command `Result`.
+- **`ui/` → Storybook story, not a component test.** Axe runs on every story; add a `play`
+  only for wiring axe can't see.
+- **Never assert on `$localize` copy.** It changes per locale/edit — assert on the
+  `Result`, the value object, or the message id.
+
+## Skeleton
+
+Co-locate `*.spec.ts` next to the unit, in the same layer folder:
+
+```
+<context>/domain/<thing>.spec.ts            # pure — no TestBed
+<context>/domain/value-objects/<vo>.spec.ts # parser: ok + normalise + each reject
+<context>/infrastructure/<x>.adapter.spec.ts# parse* trust boundary
+<context>/application/<store|command>.spec.ts
+```
+
+Minimal parser spec:
+
+```ts
+import { describe, it, expect } from 'vitest';
+import { parseThing } from './thing';
+
+describe('parseThing', () => {
+  it('accepts + normalises', () => {
+    const r = parseThing(' raw ');
+    expect(r.ok).toBe(true);
+    if (r.ok) expect(r.value).toBe('RAW');
+  });
+  it('rejects malformed', () => {
+    expect(parseThing('').ok).toBe(false); // asserts the tag, not the copy
+  });
+});
+```
+
+## Worked examples
+
+- `src/app/registratie/domain/value-objects/postcode.spec.ts` — parser style.
+- `src/app/registratie/infrastructure/brp.adapter.spec.ts` — trust boundary (`null`/`{}`).
+- `src/app/registratie/domain/registratie-wizard.machine.spec.ts` — pure reducer.
+
+## Verify
+
+```bash
+npm test                     # Vitest (ng test — no vitest.config)
+npm run test-storybook       # axe over every story (UI a11y gate)
+cd backend && dotnet test    # backend rule + endpoint + golden tests
+```