feat: add Primary message to the 3 wizard machines (RD-07)

Each wizard component re-derives the step-boundary decision the reducer
already owns: advance on a middle step, submit on the last step. This
ticket moves that decision into the machine, so RD-08 can replace the
component's guard with one dispatch.

Add a `Primary` message to each Msg union, and export a `primary(s)`
function next to the existing `next`/`submit` pair. `primary` is a
three-line branch that delegates to `next`/`submit` and writes no new
validation. Each machine tests "last step" in its own vocabulary, per
the ticket's Decisions block: `herregistratie` checks `step === 3`,
`intake` checks `currentStep(s) === 'review'`, `registratie` checks
`currentStep(s) === 'controle'`. `Next` and `Submit` stay in every
union and every reducer — `Primary` is purely additive.

Add 3 spec cases per machine (9 total): Primary advances from a
non-final step, Primary submits from the final step, and Primary is a
no-op outside the editing state. Each case also asserts the
equivalence the ticket requires for RD-08's migration:
`reduce(s, Primary)` equals `reduce(s, Next)` at a non-final step, and
equals `reduce(s, Submit)` at the final step.

Regenerate `behaviour-spec.mdx` for the 9 new `it()` titles.

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

apps/ssp/src/app/registratie/domain/registratie-wizard.machine.ts

@@ -287,6 +287,13 @@ export function submit(s: RegistratieState): RegistratieState {
   return r.ok ? { tag: 'Indienen', data: r.value } : { ...s, errors: r.error };
 }
 
+/** The primary button's action: advance, or submit from the controle step.
+    No-op outside Invullen — this is the one decision `onPrimary()` used to make. */
+export function primary(s: RegistratieState): RegistratieState {
+  if (s.tag !== 'Invullen') return s;
+  return currentStep(s) === 'controle' ? submit(s) : next(s);
+}
+
 /** Route an upload sub-message through the pure upload reducer (Invullen only). */
 export function upload(s: RegistratieState, msg: UploadMsg): RegistratieState {
   if (s.tag !== 'Invullen') return s;
@@ -312,6 +319,7 @@ export type RegistratieMsg =
   | { tag: 'Back' }
   | { tag: 'GaNaarStap'; cursor: number }
   | { tag: 'Submit' }
+  | { tag: 'Primary' }
   | { tag: 'Retry' }
   | { tag: 'SubmitConfirmed'; referentie: string }
   | { tag: 'SubmitFailed'; error: string }
@@ -342,6 +350,8 @@ export function reduce(s: RegistratieState, m: RegistratieMsg): RegistratieState
       return gaNaarStap(s, m.cursor);
     case 'Submit':
       return submit(s);
+    case 'Primary':
+      return primary(s);
     case 'Retry':
       return s.tag === 'Mislukt' ? { tag: 'Indienen', data: s.data } : s;
     case 'SubmitConfirmed':