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/app/FormBuilder/Purposes/Resolvers/EventRegistrationSubjectResolver.php

@@ -0,0 +1,24 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\FormBuilder\Purposes\Resolvers;
+
+use App\FormBuilder\Bindings\PersonProvisioner;
+use App\FormBuilder\Purposes\PurposeSubjectResolver;
+use App\Models\FormBuilder\FormSubmission;
+use App\Models\Person;
+
+/**
+ * RFC §3 Q9 — event_registration: provisions a Person via
+ * PersonProvisioner (firstOrCreate semantics). Subject may be created.
+ */
+final readonly class EventRegistrationSubjectResolver implements PurposeSubjectResolver
+{
+    public function __construct(private PersonProvisioner $provisioner) {}
+
+    public function resolveOrProvision(FormSubmission $submission): Person
+    {
+        return $this->provisioner->provisionFromSubmission($submission);
+    }
+}