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/ArtistAdvanceSubjectResolver.php

@@ -0,0 +1,46 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\FormBuilder\Purposes\Resolvers;
+
+use App\Exceptions\FormBuilder\PurposeSubjectResolutionException;
+use App\FormBuilder\Purposes\PurposeSubjectResolver;
+use App\Models\FormBuilder\FormSubmission;
+use Illuminate\Database\Eloquent\Model;
+
+/**
+ * RFC §3 Q9 — artist_advance: subject is the Artist resolved via the
+ * portal token that opened the submission. Throws if no token.
+ *
+ * The Artist model is not yet implemented (BACKLOG: ARCH-09); this
+ * resolver short-circuits when the submission already carries
+ * subject_type='artist' (set by the portal route) — the typical path.
+ * If subject is absent, throw — the route should never have allowed
+ * the submission through without a portal token.
+ */
+final readonly class ArtistAdvanceSubjectResolver implements PurposeSubjectResolver
+{
+    public function resolveOrProvision(FormSubmission $submission): Model
+    {
+        if ($submission->subject_type === 'artist' && $submission->subject_id !== null) {
+            $subject = $submission->subject;
+            if ($subject !== null) {
+                return $subject;
+            }
+            throw new PurposeSubjectResolutionException(
+                'artist_advance',
+                'subject_not_found',
+                (string) $submission->id,
+                "submission claims artist subject {$submission->subject_id} but record is gone",
+            );
+        }
+
+        throw new PurposeSubjectResolutionException(
+            'artist_advance',
+            'no_portal_token',
+            (string) $submission->id,
+            'artist_advance submission has no resolved Artist; portal token missing',
+        );
+    }
+}