feat(form-builder): FormFieldBindingService + library-to-field row copy + snapshot writer

WS-5a commit 2 of 4.

FormFieldBindingService owns all writes to the relational binding table.
Validation against config/form_binding.php entity-column registry lives here
(ARCH §6.2).

FormFieldService::insertFromLibrary now calls copyBindings instead of
hydrating JSON — the Q3 row-copy mandate. Library and field bindings share
the same table; insertion is a row-clone operation.

Snapshot writer (FormSubmissionService::buildSnapshot) serialises bindings
via toJsonShape so schema_snapshot JSON keeps its ARCH §4.6.1 / §6.3
contract. No snapshot format change.
API resources source binding output from the relational table via the same
serialiser — external shape preserved.

Tests: service transactional behaviour, copyBindings preservation,
snapshot parity, API resource parity.

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

api/tests/Feature/FormBuilder/Bindings/InsertFromLibraryCopiesBindingsTest.php

@@ -0,0 +1,66 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Feature\FormBuilder\Bindings;
+
+use App\Enums\FormBuilder\FormFieldBindingMode;
+use App\Models\FormBuilder\FormFieldBinding;
+use App\Models\FormBuilder\FormFieldLibrary;
+use App\Models\FormBuilder\FormSchema;
+use App\Models\Organisation;
+use App\Services\FormBuilder\FormFieldService;
+use Illuminate\Foundation\Testing\RefreshDatabase;
+use Tests\TestCase;
+
+final class InsertFromLibraryCopiesBindingsTest extends TestCase
+{
+    use RefreshDatabase;
+
+    public function test_insert_from_library_copies_all_bindings_and_increments_usage(): void
+    {
+        $org = Organisation::factory()->create();
+        $schema = FormSchema::factory()->create(['organisation_id' => $org->id]);
+        $library = FormFieldLibrary::factory()->create([
+            'organisation_id' => $org->id,
+            'usage_count' => 0,
+        ]);
+
+        FormFieldBinding::factory()->forLibrary($library)->entityOwned('person', 'email')->create();
+        FormFieldBinding::factory()->forLibrary($library)->mirrored('user_profile', 'bio')->create();
+
+        $service = $this->app->make(FormFieldService::class);
+        $field = $service->insertFromLibrary($schema, $library);
+
+        $copied = FormFieldBinding::query()
+            ->withoutGlobalScopes()
+            ->where('owner_type', 'form_field')
+            ->where('owner_id', $field->id)
+            ->orderBy('target_attribute')
+            ->get();
+
+        $this->assertCount(2, $copied);
+        $this->assertSame(['bio', 'email'], $copied->pluck('target_attribute')->all());
+        $this->assertSame(FormFieldBindingMode::Mirrored, $copied->firstWhere('target_attribute', 'bio')->mode);
+        $this->assertSame(FormFieldBindingMode::EntityOwned, $copied->firstWhere('target_attribute', 'email')->mode);
+
+        $this->assertSame(1, (int) $library->fresh()->usage_count);
+    }
+
+    public function test_insert_from_library_without_bindings_creates_field_without_bindings(): void
+    {
+        $org = Organisation::factory()->create();
+        $schema = FormSchema::factory()->create(['organisation_id' => $org->id]);
+        $library = FormFieldLibrary::factory()->create(['organisation_id' => $org->id]);
+
+        $service = $this->app->make(FormFieldService::class);
+        $field = $service->insertFromLibrary($schema, $library);
+
+        $this->assertSame(0, FormFieldBinding::query()
+            ->withoutGlobalScopes()
+            ->where('owner_type', 'form_field')
+            ->where('owner_id', $field->id)
+            ->count(),
+        );
+    }
+}