feat(form-builder): introduce purpose registry and definition value object

Lands the v1.0 purpose registry (WS-2 of the consolidation sprint) as a
first-class concept: a `PurposeDefinition` value object, a
`PurposeRegistry` service keyed by slug, and a declarative
`config/form_builder/purposes.php` registry with exactly the seven
purposes from ARCH-CONSOLIDATION §6.4.

Also rebuilds the morph-map in `AppServiceProvider::boot` into three
labelled blocks: (1) domain subject types derived from
`PurposeRegistry::allSubjectTypes()`, (2) non-purpose domain types
hardcoded with comments (form_schemas owner_types, activity-log
subjects), (3) framework types (spatie/activitylog; Sanctum stays
absent per addendum Q4).

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

api/app/FormBuilder/Purposes/PurposeRegistry.php

@@ -0,0 +1,85 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\FormBuilder\Purposes;
+
+use App\Enums\FormBuilder\FormSubmissionMode;
+use App\FormBuilder\Purposes\Exceptions\PurposeNotFoundException;
+use Illuminate\Contracts\Config\Repository as ConfigRepository;
+
+final class PurposeRegistry
+{
+    /** @var array<string, PurposeDefinition>|null */
+    private ?array $cache = null;
+
+    public function __construct(private readonly ConfigRepository $config) {}
+
+    /** @return array<string, PurposeDefinition> keyed by slug */
+    public function all(): array
+    {
+        if ($this->cache !== null) {
+            return $this->cache;
+        }
+
+        /** @var array<string, array<string, mixed>> $raw */
+        $raw = (array) $this->config->get('form_builder.purposes', []);
+
+        $definitions = [];
+        foreach ($raw as $slug => $attrs) {
+            $mode = $attrs['default_submission_mode'] ?? null;
+            if (! $mode instanceof FormSubmissionMode) {
+                throw new \InvalidArgumentException(
+                    "Purpose '{$slug}' has invalid default_submission_mode; expected FormSubmissionMode enum case."
+                );
+            }
+
+            $definitions[(string) $slug] = new PurposeDefinition(
+                slug: (string) $slug,
+                label: (string) ($attrs['label'] ?? ''),
+                subjectType: (string) ($attrs['subject_type'] ?? ''),
+                defaultSubmissionMode: $mode,
+                allowsPublicAccess: (bool) ($attrs['allows_public_access'] ?? false),
+                requiredBindings: array_values((array) ($attrs['required_bindings'] ?? [])),
+            );
+        }
+
+        return $this->cache = $definitions;
+    }
+
+    public function get(string $slug): PurposeDefinition
+    {
+        $all = $this->all();
+        if (! isset($all[$slug])) {
+            throw PurposeNotFoundException::forSlug($slug);
+        }
+
+        return $all[$slug];
+    }
+
+    public function has(string $slug): bool
+    {
+        return isset($this->all()[$slug]);
+    }
+
+    /** @return list<string> unique sorted list of subject_type aliases */
+    public function allSubjectTypes(): array
+    {
+        $types = array_values(array_unique(array_map(
+            static fn (PurposeDefinition $p): string => $p->subjectType,
+            $this->all(),
+        )));
+        sort($types);
+
+        return $types;
+    }
+
+    /** @return list<string> slugs with allows_public_access === true */
+    public function publicAccessibleSlugs(): array
+    {
+        return array_values(array_keys(array_filter(
+            $this->all(),
+            static fn (PurposeDefinition $p): bool => $p->allowsPublicAccess,
+        )));
+    }
+}