feat(showcase): WP-39 — linked code snippets + teaching animations

Anti-drift snippets: `?raw` isn't supported by Angular's esbuild build, so real code
is exposed via // #region showcase:<name> markers in source (registration/remote-data/
postcode/change-request.machine/intake.machine) → scripts/gen-snippets.mjs → committed
snippets.generated.ts, with a CI drift gate so the shown code is the shipped code. The
/concepts page renders the 5 real snippets (union/fold/parse/machine/steps) with a source
caption + a tiny dependency-free highlightTs (+spec); deliberately-wrong illustrations stay
authored. Teaching motion: active state node pops on transition + parse result animates in
(existing reduced-motion-safe toolkit). Generated file prettier-ignored (like api-client).

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

scripts/ci-local.sh

@@ -19,6 +19,7 @@ step "test (vitest + coverage)";  npm run test:coverage
 step "build --localize (nl+en)";  npx ng build --localize
 step "npm audit (shipped deps)";  npm audit --omit=dev
 step "backend format + tests";    ( cd backend && dotnet format BigRegister.slnx --verify-no-changes && dotnet test BigRegister.slnx )
+step "showcase snippets drift";   npm run gen:snippets && git diff --exit-code src/app/showcase/snippets.generated.ts
 step "api-client drift";          npm run gen:api && git diff --exit-code src/app/shared/infrastructure/api-client.ts backend/swagger.json
 
 if [[ "${1:-}" == "--full" ]]; then