feat(form-builder): add BindingTypeRegistry as single source of truth for target shapes (WS-6)

Config-driven mapping from (target_entity, target_attribute) to storage
shape (scalar/collection/relation), PHP type, and identity-key
eligibility. Replaces any name-suffix matching (e.g. _tags, _skills) —
those are convention-not-contract and reject by design.

Used by publish guards now and (in session 2) by FormBindingApplicator.

Refs: RFC-WS-6.md §4 (V1)

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

api/app/Providers/AppServiceProvider.php

@@ -4,6 +4,7 @@ declare(strict_types=1);
 
 namespace App\Providers;
 
+use App\FormBuilder\Bindings\BindingTypeRegistry;
 use App\FormBuilder\Purposes\PurposeRegistry;
 use App\Models\Company;
 use App\Models\CrowdList;
@@ -82,8 +83,13 @@ class AppServiceProvider extends ServiceProvider
             base_path('config/form_builder/purposes.php'),
             'form_builder.purposes',
         );
+        $this->mergeConfigFrom(
+            base_path('config/form_builder/binding_targets.php'),
+            'form_builder.binding_targets',
+        );
 
         $this->app->singleton(PurposeRegistry::class);
+        $this->app->singleton(BindingTypeRegistry::class);
 
         // Telescope is a dev-only debugging dashboard. Three-layer
         // defense keeps it out of production: composer `dont-discover`