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>

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. 510 frontend behaviours across
+**is** the suite, reshaped for a business reader. 519 frontend behaviours across
 9 contexts; 261 backend behaviours across 42 test
 classes.
 
@@ -383,6 +383,15 @@ classes.
 - gaNaarStap jumps back to an earlier step, clearing errors
 - gaNaarStap ignores a same/forward jump and jumps outside Answering
 
+#### primary
+
+- Primary advances to the next step from a non-final step
+- Primary submits from the final step
+- Primary is a no-op from a non-editing state
+- Primary advances to the next step from a non-final step
+- Primary submits from the final step
+- Primary is a no-op from a non-editing state
+
 #### reduce (message-driven happy path)
 
 - drives abroad branch end to end
@@ -605,6 +614,12 @@ classes.
 - a diploma with questions blocks Next until they are answered
 - validateAll keeps only the answers to the questions that applied
 
+#### primary
+
+- Primary advances to the next step from a non-final step
+- Primary submits from the final step
+- Primary is a no-op from a non-editing state
+
 #### reduce (message-driven happy path)
 
 - adres and correspondentie set, Next advances from adres to beroep