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/kernel/fp.ts

@@ -0,0 +1,23 @@
+/**
+ * Tiny native-TS functional toolkit. No dependency — this is the whole "library".
+ * Reused by every "impossible states" concept in the POC.
+ */
+
+/** Exhaustiveness guard: put in the `default` arm of a union switch. Adding a
+    new variant without handling it then fails to compile (x is no longer never). */
+export function assertNever(x: never): never {
+  throw new Error('Unexpected variant: ' + JSON.stringify(x));
+}
+
+/** A computation that either succeeded with a value or failed with an error.
+    Plain objects (no classes) to match the signal/httpResource ergonomics. */
+export type Result<E, T> =
+  | { readonly ok: true; readonly value: T }
+  | { readonly ok: false; readonly error: E };
+
+export const ok = <T>(value: T): Result<never, T> => ({ ok: true, value });
+export const err = <E>(error: E): Result<E, never> => ({ ok: false, error });
+
+/** Nominal typing: Brand<string, 'Postcode'> is assignable from a plain string
+    only through an explicit cast — so a smart constructor is the only minter. */
+export type Brand<T, B extends string> = T & { readonly __brand: B };