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>

.github/workflows/ci.yml

@@ -29,6 +29,8 @@ jobs:
       - run: npm run lint
       # Bounded-context + atomic-layer boundaries (WP-38, dependency-cruiser).
       - run: npm run dep:check
+      # Showcase snippets must match their real source regions (WP-39, no drift).
+      - run: npm run gen:snippets && git diff --exit-code src/app/showcase/snippets.generated.ts
       - run: npm run format:check
       - run: npm run check:tokens
       # Runs the full suite AND reports coverage (WP-46, report-only — no thresholds, so