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/herregistratie/domain/intake.machine.ts

@@ -54,8 +54,10 @@ export function lageUren(a: Answers, scholingThreshold = SCHOLING_THRESHOLD_DEFA
   return r.ok && r.value < scholingThreshold;
 }
 
+// #region showcase:steps
 /** The fixed step list. Number of steps never changes; questions reveal inline. */
 export const STEPS: StepId[] = ['buitenland', 'werk', 'review'];
+// #endregion showcase:steps
 
 /** Per-field error map: one message per question, since a step holds several. */
 type Errors = Partial<Record<keyof Answers, string>>;