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