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/registratie/domain/registratie-wizard.machine.ts

@@ -66,7 +66,7 @@ export type DraftField = 'straat' | 'postcode' | 'woonplaats' | 'email';
 
 /** Per-field error map. `antwoorden` holds per-policy-question errors, keyed by
     question id (a step can show several questions). */
-export interface Errors {
+export type Errors = {
   straat?: string;
   postcode?: string;
   woonplaats?: string;
@@ -75,7 +75,7 @@ export interface Errors {
   diploma?: string;
   documenten?: string;
   antwoorden?: Record<string, string>;
-}
+};
 
 export type RegistratieState =
   | { tag: 'Invullen'; draft: Draft; cursor: number; errors: Errors; upload: UploadState }