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/PurposeDefinition.php

@@ -0,0 +1,25 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\FormBuilder\Purposes;
+
+use App\Enums\FormBuilder\FormSubmissionMode;
+
+final readonly class PurposeDefinition
+{
+    /**
+     * @param  list<string>  $requiredBindings  Binding paths in `{entity}.{attribute}` form
+     *                                          (Pattern A notation used in `form_fields.binding` JSON).
+     *                                          Consumed by the pre-publish check (FormSchemaService::publish)
+     *                                          and, after WS-5/WS-6, by `form_field_bindings`.
+     */
+    public function __construct(
+        public string $slug,
+        public string $label,
+        public string $subjectType,
+        public FormSubmissionMode $defaultSubmissionMode,
+        public bool $allowsPublicAccess,
+        public array $requiredBindings,
+    ) {}
+}