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>

src/docs/i18n.mdx

@@ -0,0 +1,93 @@
+import { Meta } from '@storybook/addon-docs/blocks';
+
+<Meta title="Foundations/Internationalization" />
+
+# Internationalization (the locale seam)
+
+Every user-visible string goes through Angular's first-party **`$localize`** — no
+third-party i18n library. The source locale is **`nl`**; a second locale is a
+**translation file, not a code change**. That's the seam: adding English touched
+`src/locale/messages.en.xlf`, not the components.
+
+## How it's wired
+
+| Piece | Where | What |
+| --- | --- | --- |
+| Source locale | `angular.json` → `i18n.sourceLocale` | `nl` — the language the code is written in |
+| Locales | `angular.json` → `i18n.locales.en` | points at `src/locale/messages.en.xlf` |
+| Missing-translation policy | `angular.json` → `i18nMissingTranslation` | `error` — a missing `<target>` fails the build |
+| Runtime global | `angular.json` → `polyfills` | `@angular/localize/init` provides `$localize` |
+| English build/serve | `angular.json` → `configurations.en` | `ng build --configuration=en`, `ng serve --configuration=en` |
+
+Locale switching is **build-time**, not runtime: each locale is its own bundle. There is
+no in-app language picker (out of scope for the POC).
+
+## Authoring copy
+
+Two forms, same custom-id rule. The id is **stable** and shaped `@@<context>.<key>`, so
+translations survive copy edits.
+
+**In TS logic / value objects — tagged template:**
+
+```ts
+// src/app/registratie/domain/value-objects/postcode.ts
+return err($localize`:@@validation.postcode:Voer een geldige postcode in, bijv. 1234 AB.`);
+```
+
+With placeholders (named, so translators can reorder):
+
+```ts
+$localize`:@@aanvraag.row.ingediend:ingediend op ${formatDatumNl(a.submittedAt)}:datum:`;
+```
+
+**In inline component templates — the `i18n` attribute:**
+
+```html
+<!-- src/app/auth/ui/login-form/login-form.component.ts -->
+<app-button type="submit" i18n="@@login.submit">Inloggen met DigiD</app-button>
+```
+
+**Shared/English components never hardcode Dutch.** They expose copy as `input()`s with
+localizable defaults; the domain caller may override. See
+`shared/ui/async/async.component.ts`:
+
+```ts
+errorText = input($localize`:@@async.error:Er ging iets mis bij het laden van de gegevens.`);
+```
+
+## Extract & translate loop
+
+```bash
+npm run extract-i18n   # ng extract-i18n → src/locale/messages.xlf (source, nl)
+```
+
+Then a translator fills `<target>`s in `src/locale/messages.en.xlf`. Both files carry the
+same trans-units (currently 690 = 690, no drift); the `.en.xlf` header is
+`source-language="nl" target-language="en"`. Because `i18nMissingTranslation: error`, a
+forgotten target breaks the `en` build rather than silently shipping Dutch.
+
+## Testing languages without coupling to the strings
+
+**Rule: never assert on rendered copy.** Copy is the thing that changes per locale and per
+edit — a test that reads `"Voer een geldige postcode in"` breaks the moment a translator or
+a product owner touches the wording, in every locale. Assert on what's *invariant* instead:
+
+- **Parsers / value objects** — assert on the `Result` discriminant and the parsed value,
+  not the error message. This is the existing house pattern
+  (`registratie/domain/value-objects/postcode.spec.ts`):
+
+  ```ts
+  expect(parsePostcode('0234AB').ok).toBe(false); // rejects — never inspects the $localize string
+  ```
+
+- **The seam itself** — if you must verify that translation works, check that a known
+  **id flips**, not that a specific phrase appears. Build/serve the `en` configuration and
+  confirm the target for a stable id renders, e.g. `login.submit`: `nl` "Inloggen met
+  DigiD" → `en` "Log in with DigiD". You're testing the wiring, not the wording.
+
+  ```bash
+  ng serve --configuration=en   # then eyeball, or point an e2e at the en bundle
+  ```
+
+See [Testing strategy](?path=/docs/foundations-testing-strategy--docs) for how this fits the
+rest of the test pyramid.