feat(fp): WP-20 — second locale proof (nl/en build seam)

angular.json gains an i18n block (sourceLocale nl, en translation file) and
an `en` build/serve configuration with i18nMissingTranslation: "error" so a
new $localize string without an English unit fails the build, not silently
falls back. CI now runs `ng build --localize` to build both locales every
run. Verified end-to-end, not just "the build succeeded": the nl bundle
ships "Inloggen met DigiD", the en bundle ships "Log in with DigiD".

Incidental: prettier/compodoc regen noise in docs/wcag-checklist.md,
src/docs/a11y.mdx, documentation.json from the same working session.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -173,6 +173,14 @@ degrade to an instant navigation.
   **dev-only** — it is not wired into production builds.
 - `.npmrc` sets `legacy-peer-deps=true` because `@storybook/angular`'s peer range lags
   Angular 22; the builder runs fine (build verified).
+- **i18n**: every user-facing string is `$localize`-wrapped with a stable `@@id`
+  (source locale `nl`). `npx ng build --localize` (CI runs this) builds both `nl` and
+  `en` — a genuine second-locale build, not just an unexercised claim — into
+  `dist/atomic-design-poc/browser/{nl,en}/`; `ng serve --configuration=en` serves the
+  English build locally. `src/locale/messages.en.xlf` is real (if demo-quality)
+  English, not machine-untranslated placeholders; `angular.json`'s
+  `i18nMissingTranslation: "error"` fails the build if a new `$localize` string ships
+  without a translation. `npm run extract-i18n` regenerates the `nl` reference file.
 
 ### Dependency security
 
@@ -186,6 +194,9 @@ We do **not** run `npm audit fix --force`: its proposed fix downgrades Angular 2
 
 ### Deliberately out of scope (POC)
 
-Real auth/DigiD, real BRP/DUO upstreams, a database/persisted audit store, i18n,
-NgRx, licensed RO/Rijks fonts + logo (system-font stack; text wordmark). (The backend
-itself _is_ implemented.)
+Real auth/DigiD, real BRP/DUO upstreams, a database/persisted audit store, NgRx,
+licensed RO/Rijks fonts + logo (system-font stack; text wordmark). (The backend
+itself _is_ implemented.) i18n's build seam is proven (see above) but
+production-quality translation, a runtime locale switcher, and RTL/pluralization
+edge cases are not — the `en` file is demo-quality, and locale is a build-time
+choice, not a switch in the running app.