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>

src/app/shared/application/remote-data.ts

@@ -27,6 +27,7 @@ export function fromResource<T>(
   return { tag: 'Loading' };
 }
 
+// #region showcase:fold
 /** Exhaustive fold: you must handle every case, checked at compile time. */
 export function foldRemote<E, T, R>(
   rd: RemoteData<E, T>,
@@ -42,9 +43,10 @@ export function foldRemote<E, T, R>(
     case 'Success':
       return h.success(rd.value);
     default:
-      return assertNever(rd);
+      return assertNever(rd); // add a variant → compile error until handled
   }
 }
+// #endregion showcase:fold
 
 // --- Combinators -----------------------------------------------------------
 // Let several independent async sources be treated as one. When you combine