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>

package.json

@@ -19,6 +19,7 @@
     "check:tokens": "bash scripts/check-tokens.sh",
     "dep:check": "depcruise src/app --config .dependency-cruiser.js",
     "dep:graph": "bash scripts/dep-graph.sh",
+    "gen:snippets": "node scripts/gen-snippets.mjs",
     "ci": "bash scripts/ci-local.sh",
     "e2e": "playwright test",
     "extract-i18n": "ng extract-i18n --output-path src/locale"