refactor(engine): mover limpieza de instancias y validación required al motor [4E]

applyAnswer y isRequiredAndUnanswered pasan de wizard-form.tsx al motor
como funciones puras — eran lógica de flujo en el presentador (viola ADR-005).
Limpieza de respuestas huérfanas al remover un vínculo y validación required
por instancia, ahora testeables. 20/20 tests. Verificado end-to-end con navegador.
4E sin construcción pendiente: el mini-wizard es propiedad emergente del motor.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

components/wizard-form.tsx

@@ -4,12 +4,13 @@ import type { BrandConfig, Survey } from '@/lib/types'
 import type { AnswerValue, FormAnswers, FormSchema, Screen } from '@/lib/form-engine.types'
 import {
   answerKey,
+  applyAnswer,
   expandInstances,
   initialState,
+  isRequiredAndUnanswered,
   next,
   back,
   resolveScreens,
-  setAnswer,
 } from '@/lib/form-engine'
 import { loadDraft, saveDraft } from '@/lib/storage'
 import { ConsentScreen } from './consent-screen'
@@ -137,20 +138,7 @@ export function WizardForm({ survey, schema }: WizardFormProps) {
     const code = screen.question.code
     const instanceId = screen.instanceId ?? undefined
 
-    setState((prev) => {
-      const updated = setAnswer(prev, code, value, instanceId)
-      // Limpia respuestas de pantallas que dejaron de ser visibles tras este cambio.
-      const before = resolveScreens(schema, prev.answers)
-      const after = resolveScreens(schema, updated.answers)
-      const afterIds = new Set(after.map((s) => s.id))
-      const disappeared = before.filter((s) => !afterIds.has(s.id) && s.question)
-      if (disappeared.length === 0) return updated
-      const clearedAnswers = { ...updated.answers }
-      for (const s of disappeared) {
-        delete clearedAnswers[answerKey(s.question!.code, s.instanceId ?? undefined)]
-      }
-      return { ...updated, answers: clearedAnswers }
-    })
+    setState((prev) => applyAnswer(prev, schema, code, value, instanceId))
 
     setErrors((prev) => {
       if (!prev[code]) return prev
@@ -160,7 +148,7 @@ export function WizardForm({ survey, schema }: WizardFormProps) {
   }
 
   function handleNext() {
-    if (screen.kind === 'question' && screen.question?.required && isEmpty(currentValue)) {
+    if (isRequiredAndUnanswered(screen, state.answers)) {
       setErrors((e) => ({ ...e, [screen.question!.code]: 'Este campo es obligatorio.' }))
       return
     }