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>

apps/ssp/src/app/herregistratie/ui/herregistratie-wizard/herregistratie-wizard.component.ts

@@ -9,6 +9,7 @@ import {
   WizardPhase,
   naarStapLabel,
 } from '@shared/layout/wizard-shell/wizard-shell.component';
+import { toWizardErrors } from '@shared/layout/wizard-shell/wizard-errors';
 import { ConfirmationComponent } from '@shared/ui/confirmation/confirmation.component';
 import { createStore } from '@shared/application/store';
 import { whenTag } from '@shared/kernel/fp';
@@ -251,12 +252,7 @@ export class HerregistratieWizardComponent {
     }
   });
   /** Current step's field errors, flattened for the shell's error summary. */
-  protected errorList = computed<WizardError[]>(() => {
-    const e = this.editing()?.errors ?? {};
-    return (Object.keys(e) as (keyof typeof e)[])
-      .filter((k) => e[k])
-      .map((k) => ({ id: k, message: e[k]! }));
-  });
+  protected errorList = computed<WizardError[]>(() => toWizardErrors(this.editing()?.errors ?? {}));
 
   constructor() {
     // An explicit seed (stories/tests) wins; otherwise resume the backend draft