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/herregistratie/domain/intake.machine.spec.ts

@@ -10,6 +10,7 @@ import {
   back,
   gaNaarStap,
   submit,
+  primary,
   resolve,
   reduce,
   IntakeState,
@@ -207,6 +208,33 @@ describe('submit', () => {
   });
 });
 
+describe('primary', () => {
+  // Same fixture as the 'submit' describe block above: buitenland answered 'nee',
+  // uren high enough to skip the scholing question.
+  const highUren = givenIntake(
+    { tag: 'SetAnswer', key: 'buitenlandGewerkt', value: 'nee' },
+    { tag: 'SetAnswer', key: 'uren', value: '4160' },
+  );
+
+  it('Primary advances to the next step from a non-final step', () => {
+    expect(currentStep(expectTag(highUren, 'Answering'))).toBe('buitenland'); // not the final step
+    expect(reduce(highUren, { tag: 'Primary' })).toEqual(reduce(highUren, { tag: 'Next' }));
+    expect(expectTag(primary(highUren), 'Answering').cursor).toBe(1);
+  });
+
+  it('Primary submits from the final step', () => {
+    const atReview = reduce(reduce(highUren, { tag: 'Next' }), { tag: 'Next' });
+    expect(currentStep(expectTag(atReview, 'Answering'))).toBe('review'); // the final step
+    expect(reduce(atReview, { tag: 'Primary' })).toEqual(reduce(atReview, { tag: 'Submit' }));
+    expect(primary(atReview).tag).toBe('Submitting');
+  });
+
+  it('Primary is a no-op from a non-editing state', () => {
+    const submitting = submit(reduce(reduce(highUren, { tag: 'Next' }), { tag: 'Next' }));
+    expect(primary(submitting)).toBe(submitting);
+  });
+});
+
 describe('reduce (message-driven happy path)', () => {
   it('drives abroad branch end to end', () => {
     let s: IntakeState = initial;