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/PublishGuardResult.php

@@ -0,0 +1,47 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\FormBuilder\Publishing;
+
+/**
+ * RFC-WS-6 §3 (Q13) — outcome of evaluating a single {@see PublishGuard}.
+ * Sealed via two named constructors: callers cannot construct
+ * inconsistent states (passed-with-message, etc.).
+ */
+final readonly class PublishGuardResult
+{
+    /**
+     * @param  array<string, mixed>  $context
+     */
+    private function __construct(
+        public string $guardCode,
+        public bool $passed,
+        public ?string $messageKey = null,
+        public ?string $offendingFormFieldId = null,
+        public array $context = [],
+    ) {}
+
+    public static function passed(string $guardCode): self
+    {
+        return new self(guardCode: $guardCode, passed: true);
+    }
+
+    /**
+     * @param  array<string, mixed>  $context
+     */
+    public static function failed(
+        string $guardCode,
+        string $messageKey,
+        ?string $offendingFormFieldId = null,
+        array $context = [],
+    ): self {
+        return new self(
+            guardCode: $guardCode,
+            passed: false,
+            messageKey: $messageKey,
+            offendingFormFieldId: $offendingFormFieldId,
+            context: $context,
+        );
+    }
+}