feat(form-builder): add PurposeSubjectResolver per purpose (WS-6)

Parallel interface to PurposeGuardProvider for runtime subject
resolution. Seven concrete resolvers, one per v1.0 purpose. Wired
through purposes.php via subject_resolver_class key.

EventRegistration uses PersonProvisioner (may create). Other purposes
resolve from existing context (portal token, production request, auth).
IncidentReport is the only purpose allowed to return null (anonymous-
allowed configurations); the others return concrete model types
(narrowed via PHP covariance) for caller convenience.

Refs: RFC-WS-6.md §3 (Q9)

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

api/tests/Feature/FormBuilder/Purposes/AllPurposesSubjectResolverWiringTest.php

@@ -0,0 +1,37 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Feature\FormBuilder\Purposes;
+
+use App\FormBuilder\Purposes\PurposeRegistry;
+use App\FormBuilder\Purposes\PurposeSubjectResolver;
+use Tests\TestCase;
+
+final class AllPurposesSubjectResolverWiringTest extends TestCase
+{
+    public function test_every_purpose_has_a_subject_resolver_class(): void
+    {
+        /** @var array<string, mixed> $config */
+        $config = config('form_builder.purposes');
+        $this->assertNotEmpty($config);
+
+        foreach ($config as $slug => $attrs) {
+            $this->assertArrayHasKey(
+                'subject_resolver_class',
+                $attrs,
+                "Purpose '{$slug}' is missing the subject_resolver_class config key.",
+            );
+        }
+    }
+
+    public function test_registry_resolves_a_subject_resolver_for_every_purpose(): void
+    {
+        $registry = $this->app->make(PurposeRegistry::class);
+
+        foreach (array_keys($registry->all()) as $slug) {
+            $resolver = $registry->subjectResolverFor($slug);
+            $this->assertInstanceOf(PurposeSubjectResolver::class, $resolver);
+        }
+    }
+}