Restructure into DDD bounded contexts + functional state management

Reorganise from atomic-design-only folders into bounded contexts
(auth / registratie / herregistratie) over a shared kernel, each split into
domain / application / infrastructure / ui layers. Dependencies point inward;
the domain layer is framework-free. Path aliases (@shared/@auth/@registratie/
@herregistratie) make import direction explicit.

State management (Elm-style, native TS, no new deps):
- shared/application/store.ts — createStore(init, update): pure reducer + signal
- shared/application/remote-data.ts — add map/map2/map3/andThen combinators so
  several services fold into one RemoteData; <app-async> gains an [rd] input
- registratie/application/big-profile.store.ts — root singleton combining the
  BIG-register and BRP services via map2 into one state; holds the optimistic
  herregistratie flag shared with the dashboard
- herregistratie: machine gains a WizardMsg union + pure reduce; submit is a
  command that calls infra and dispatches the result, with optimistic update +
  rollback against the shared store
- auth: SessionStore + DigiD adapter + functional route guard; login establishes
  the session, protected routes use canActivate

Rich domain: registration.policy.ts (statusColor/label, herregistratie
eligibility, invariants); BigNummer/Postcode/Uren value objects with smart
constructors. status-badge is now domain-free (colour/label inputs).

Specs for the reducer, RemoteData combinators, and eligibility policy.

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

src/app/shared/layout/page-shell/page-shell.component.ts

@@ -0,0 +1,31 @@
+import { Component, input } from '@angular/core';
+import { HeadingComponent } from '@shared/ui/heading/heading.component';
+import { LinkComponent } from '@shared/ui/link/link.component';
+
+/** Template: standard page body — optional back-link, a heading, optional intro,
+    and projected content. Rendered inside the persistent ShellComponent via the
+    router outlet, so it owns only the content (not the header/footer chrome). */
+@Component({
+  selector: 'app-page-shell',
+  imports: [HeadingComponent, LinkComponent],
+  styles: [':host{display:block}'],
+  template: `
+    <div [style.max-width]="width() === 'narrow' ? '32rem' : null">
+      @if (backLink()) {
+        <p><app-link [to]="backLink()!">← {{ backLabel() }}</app-link></p>
+      }
+      <app-heading [level]="1">{{ heading() }}</app-heading>
+      @if (intro()) {
+        <p class="rhc-paragraph" style="margin-bottom:1.5rem">{{ intro() }}</p>
+      }
+      <ng-content />
+    </div>
+  `,
+})
+export class PageShellComponent {
+  heading = input.required<string>();
+  intro = input<string>();
+  backLink = input<string>();
+  backLabel = input('Terug naar overzicht');
+  width = input<'default' | 'narrow'>('default');
+}