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/SchemaSnapshotEmbedsBindingFromRelationalTableTest.php

@@ -0,0 +1,119 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Feature\FormBuilder\Bindings;
+
+use App\Enums\FormBuilder\FormFieldBindingMode;
+use App\Enums\FormBuilder\FormFieldType;
+use App\Enums\FormBuilder\FormPurpose;
+use App\Enums\FormBuilder\FormSchemaSnapshotMode;
+use App\Models\FormBuilder\FormField;
+use App\Models\FormBuilder\FormFieldBinding;
+use App\Models\FormBuilder\FormSchema;
+use App\Models\FormBuilder\FormSubmission;
+use App\Models\Organisation;
+use App\Models\User;
+use App\Services\FormBuilder\FormSubmissionService;
+use Illuminate\Foundation\Testing\RefreshDatabase;
+use Tests\TestCase;
+
+/**
+ * Asserts the `schema_snapshot` JSON keeps its ARCH §4.6.1 / §6.3 shape
+ * after WS-5a: the snapshot writer now serialises bindings from the
+ * relational table via FormFieldBindingService::toJsonShape, but the
+ * per-field JSON embedded in the snapshot is byte-for-byte the pre-WS-5a
+ * shape for entity-owned and mirrored patterns.
+ */
+final class SchemaSnapshotEmbedsBindingFromRelationalTableTest extends TestCase
+{
+    use RefreshDatabase;
+
+    public function test_snapshot_embeds_entity_owned_binding_in_arch_shape(): void
+    {
+        $schema = $this->schemaWithSnapshot();
+
+        $field = FormField::factory()->create([
+            'form_schema_id' => $schema->id,
+            'field_type' => FormFieldType::EMAIL->value,
+            'slug' => 'email',
+            'label' => 'E-mail',
+        ]);
+        FormFieldBinding::factory()->forField($field)->entityOwned('person', 'email')->create();
+
+        $submission = $this->submitFor($schema);
+        $snapshot = $submission->fresh()->schema_snapshot;
+
+        $this->assertIsArray($snapshot);
+        $embedded = collect($snapshot['fields'])->firstWhere('id', $field->id);
+        $this->assertNotNull($embedded);
+        $this->assertSame([
+            'mode' => 'entity_owned',
+            'entity' => 'person',
+            'column' => 'email',
+        ], $embedded['binding']);
+    }
+
+    public function test_snapshot_embeds_mirrored_binding_with_sync_direction(): void
+    {
+        $schema = $this->schemaWithSnapshot();
+
+        $field = FormField::factory()->create([
+            'form_schema_id' => $schema->id,
+            'field_type' => FormFieldType::TEXT->value,
+            'slug' => 'noodcontact',
+            'label' => 'Noodcontact',
+        ]);
+        FormFieldBinding::factory()->forField($field)->mirrored('user_profile', 'emergency_contact_name')->create();
+
+        $submission = $this->submitFor($schema);
+        $snapshot = $submission->fresh()->schema_snapshot;
+
+        $embedded = collect($snapshot['fields'])->firstWhere('id', $field->id);
+        $this->assertSame([
+            'mode' => 'mirrored',
+            'entity' => 'user_profile',
+            'column' => 'emergency_contact_name',
+            'sync_direction' => 'write_on_submit',
+        ], $embedded['binding']);
+    }
+
+    public function test_snapshot_embeds_null_binding_for_form_owned_field(): void
+    {
+        $schema = $this->schemaWithSnapshot();
+
+        $field = FormField::factory()->create([
+            'form_schema_id' => $schema->id,
+            'field_type' => FormFieldType::TEXT->value,
+            'slug' => 'motivatie',
+            'label' => 'Motivatie',
+        ]);
+
+        $submission = $this->submitFor($schema);
+        $snapshot = $submission->fresh()->schema_snapshot;
+
+        $embedded = collect($snapshot['fields'])->firstWhere('id', $field->id);
+        $this->assertNull($embedded['binding']);
+    }
+
+    private function schemaWithSnapshot(): FormSchema
+    {
+        $org = Organisation::factory()->create();
+
+        return FormSchema::factory()->create([
+            'organisation_id' => $org->id,
+            'purpose' => FormPurpose::USER_PROFILE->value,
+            'snapshot_mode' => FormSchemaSnapshotMode::ALWAYS->value,
+        ]);
+    }
+
+    private function submitFor(FormSchema $schema): FormSubmission
+    {
+        $user = User::factory()->create();
+        $service = $this->app->make(FormSubmissionService::class);
+
+        $submission = $service->createDraft($schema, $user, $user);
+
+        return $service->submit($submission, $user);
+    }
+}