feat(form-builder): add PublishGuard framework + 9 concrete guards (WS-6)

Per-purpose schema validation composes a PurposeGuardProvider returning
a list of guards. Errors collected (not first-fail) so the builder UI
surfaces every issue per save. ConditionalRequirement composes higher-
order without proliferating one-off classes.

RequiresIdentityKeyBinding checks the is_identity_key flag specifically;
the binding-existence check is handled additively by the existing
assertRequiredBindingsPresent in FormSchemaService.

SchemaHasLinkedEvent checks owner_type='event' + owner_id (FormSchema
uses polymorphic owner; there is no direct event_id column).

i18n messages live in lang/nl/form_builder_publish_guards.php.

Refs: RFC-WS-6.md §3 (Q13), §4 (V1, V3)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

api/app/FormBuilder/Publishing/RequiresIdentityKeyBinding.php

@@ -0,0 +1,49 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\FormBuilder\Publishing;
+
+use App\Models\FormBuilder\FormField;
+use App\Models\FormBuilder\FormFieldBinding;
+use App\Models\FormBuilder\FormSchema;
+
+/**
+ * RFC-WS-6 §3 (Q13) — given that the binding's existence is enforced
+ * upstream by FormSchemaService::assertRequiredBindingsPresent(), this
+ * guard adds the `is_identity_key=true` flag check.
+ */
+final readonly class RequiresIdentityKeyBinding implements PublishGuard
+{
+    public function __construct(
+        private string $entity,
+        private string $attribute,
+    ) {}
+
+    public function code(): string
+    {
+        return "requires_identity_key_binding:{$this->entity}:{$this->attribute}";
+    }
+
+    public function evaluate(FormSchema $schema): PublishGuardResult
+    {
+        /** @var FormField $field */
+        foreach ($schema->fields as $field) {
+            /** @var FormFieldBinding $binding */
+            foreach ($field->bindings as $binding) {
+                if ($binding->target_entity === $this->entity
+                    && $binding->target_attribute === $this->attribute
+                    && (bool) $binding->is_identity_key
+                ) {
+                    return PublishGuardResult::passed($this->code());
+                }
+            }
+        }
+
+        return PublishGuardResult::failed(
+            guardCode: $this->code(),
+            messageKey: 'form_builder_publish_guards.requires_identity_key_binding',
+            context: ['entity' => $this->entity, 'attribute' => $this->attribute],
+        );
+    }
+}