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/Exceptions/FormBuilder/PurposeSubjectResolutionException.php

@@ -0,0 +1,29 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\FormBuilder;
+
+use RuntimeException;
+
+/**
+ * RFC-WS-6 §3 (Q9) — failure during PurposeSubjectResolver operation.
+ *
+ * Reason codes (informal contract):
+ *   - 'no_portal_token'      artist_advance has no portal context
+ *   - 'no_production_request' supplier_intake has no production_request link
+ *   - 'no_auth'              auth-based purpose has no authenticated user
+ *   - 'no_person_for_user'   user has no linked Person
+ *   - 'subject_not_found'    subject_type/subject_id set but record gone
+ */
+final class PurposeSubjectResolutionException extends RuntimeException
+{
+    public function __construct(
+        public readonly string $purposeSlug,
+        public readonly string $reasonCode,
+        public readonly string $submissionId,
+        ?string $message = null,
+    ) {
+        parent::__construct($message ?? "Subject resolution failed for {$purposeSlug}: {$reasonCode} (submission {$submissionId})");
+    }
+}