feat: build the questionnaire form from its definition

src/Model/Questionnaire/Definition.php

@@ -41,6 +41,30 @@ readonly class Definition
         return $this->getQuestions()[$key] ?? null;
     }
 
+    /**
+     * The answers worth keeping once the questionnaire is submitted: those to questions that are
+     * shown, and not empty. A draft keeps everything, so switching an answer back restores the
+     * follow-up the respondent had already written.
+     *
+     * @param array<string, mixed> $answers keyed by question key
+     *
+     * @return array<string, mixed>
+     */
+    public function withoutHiddenAnswers(array $answers): array
+    {
+        $kept = [];
+
+        foreach ($this->getQuestions() as $key => $question) {
+            $answer = $answers[$key] ?? null;
+
+            if (null !== $answer && '' !== $answer && [] !== $answer && $this->isVisible($question, $answers)) {
+                $kept[$key] = $answer;
+            }
+        }
+
+        return $kept;
+    }
+
     /**
      * Whether a question is shown for these answers: it has no show_if, or its condition is met
      * and the question it depends on is shown in turn. A hidden question's answer counts for