refactor: extract toWizardErrors, adopted by all 3 wizards (RD-20)

Each wizard flattened its per-field error record into the shell's
WizardError[] summary with its own copy of the same loop. Extract one
pure helper, wizard-errors.ts, next to naarStapLabel. Add a spec that
covers a flat record, an empty record, skipped undefined/empty-string
values, the idPrefix, and a skipped nested object.

registratie-wizard.machine.ts changes Errors from an interface to a
type alias, because only a type alias gets an implicit index
signature and is assignable to the helper's Record<string, unknown>
parameter. The other two machines already declare their error maps as
type aliases, so this also makes the three consistent.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

libs/shared/docs/behaviour-spec.mdx

@@ -20,7 +20,7 @@ tested where._
 
 Every bullet below is a real test name from the suite — an `it()` title (frontend) or a test
 method name (backend), read as a sentence. Nothing here is hand-written prose: this page
-**is** the suite, reshaped for a business reader. 532 frontend behaviours across
+**is** the suite, reshaped for a business reader. 538 frontend behaviours across
 9 contexts; 261 backend behaviours across 42 test
 classes.
 
@@ -973,6 +973,15 @@ classes.
 - unwraps a Success value
 - is the fallback for every other state
 
+#### toWizardErrors
+
+- flattens a flat record of field errors
+- returns an empty list for an empty record
+- skips a value that is undefined
+- skips a value that is an empty string
+- prefixes every id with idPrefix when given
+- skips a value that is a nested object, so the caller can flatten it separately
+
 #### upload lifecycle messages
 
 - queued → progress → complete

libs/shared/src/layout/wizard-shell/wizard-errors.spec.ts

@@ -0,0 +1,35 @@
+import { describe, it, expect } from 'vitest';
+import { toWizardErrors } from './wizard-errors';
+
+describe('toWizardErrors', () => {
+  it('flattens a flat record of field errors', () => {
+    expect(toWizardErrors({ straat: 'Verplicht', postcode: 'Ongeldig' })).toEqual([
+      { id: 'straat', message: 'Verplicht' },
+      { id: 'postcode', message: 'Ongeldig' },
+    ]);
+  });
+
+  it('returns an empty list for an empty record', () => {
+    expect(toWizardErrors({})).toEqual([]);
+  });
+
+  it('skips a value that is undefined', () => {
+    expect(toWizardErrors({ straat: undefined })).toEqual([]);
+  });
+
+  it('skips a value that is an empty string', () => {
+    expect(toWizardErrors({ straat: '' })).toEqual([]);
+  });
+
+  it('prefixes every id with idPrefix when given', () => {
+    expect(toWizardErrors({ q1: 'Verplicht' }, 'vraag-')).toEqual([
+      { id: 'vraag-q1', message: 'Verplicht' },
+    ]);
+  });
+
+  it('skips a value that is a nested object, so the caller can flatten it separately', () => {
+    expect(toWizardErrors({ straat: 'Verplicht', antwoorden: { q1: 'Verplicht' } })).toEqual([
+      { id: 'straat', message: 'Verplicht' },
+    ]);
+  });
+});

libs/shared/src/layout/wizard-shell/wizard-errors.ts

@@ -0,0 +1,13 @@
+import type { WizardError } from './wizard-shell.component';
+
+/** Flatten a machine's per-field error record into the shell's summary list.
+    Values that are not a non-empty string are skipped, so a nested group
+    (the registratie wizard's `antwoorden`) is appended by the caller with its
+    own `idPrefix` rather than special-cased here. */
+export function toWizardErrors(errors: Record<string, unknown>, idPrefix = ''): WizardError[] {
+  const out: WizardError[] = [];
+  for (const [k, v] of Object.entries(errors)) {
+    if (typeof v === 'string' && v) out.push({ id: idPrefix + k, message: v });
+  }
+  return out;
+}