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/ConditionalRequirement.php

@@ -0,0 +1,52 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\FormBuilder\Publishing;
+
+use App\Models\FormBuilder\FormSchema;
+use Closure;
+
+/**
+ * RFC-WS-6 §3 (Q13) — higher-order composer. Runs the predicate first;
+ * delegates to the sub-guard only if the predicate returns true.
+ * Otherwise returns passed().
+ */
+final readonly class ConditionalRequirement implements PublishGuard
+{
+    /**
+     * @param  Closure(FormSchema): bool  $predicate
+     */
+    public function __construct(
+        private Closure $predicate,
+        private PublishGuard $subGuard,
+        private string $code,
+    ) {}
+
+    public function code(): string
+    {
+        return "conditional:{$this->code}";
+    }
+
+    public function evaluate(FormSchema $schema): PublishGuardResult
+    {
+        if (! ($this->predicate)($schema)) {
+            return PublishGuardResult::passed($this->code());
+        }
+
+        $subResult = $this->subGuard->evaluate($schema);
+        if ($subResult->passed) {
+            return PublishGuardResult::passed($this->code());
+        }
+
+        return PublishGuardResult::failed(
+            guardCode: $this->code(),
+            messageKey: $subResult->messageKey ?? 'form_builder_publish_guards.conditional',
+            offendingFormFieldId: $subResult->offendingFormFieldId,
+            context: array_merge(
+                $subResult->context,
+                ['delegated_to' => $this->subGuard->code()],
+            ),
+        );
+    }
+}