feat(form-builder): MergeStrategy enum methods + binding value objects (WS-6)

- FormFieldBindingMergeStrategy::nullWinnerBehaviour() and
  isValidForScalarTargets() encode the per-strategy null-winner matrix
  (RFC Q7) and the collection-only restriction (RFC V1).
- ResolvedBinding/BindingApplicationResult/BindingPassResult readonly
  DTOs for the binding pipeline. Construction-time validation for
  trust level. Apply-status derived from result aggregate.

Note: the existing enum is named FormFieldBindingMergeStrategy (not
MergeStrategy as the prompt sketched). Methods added to it directly.

Refs: RFC-WS-6.md §3 (Q4, Q7), §4 (V1)

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

api/tests/Unit/FormBuilder/Bindings/BindingApplicationResultTest.php

@@ -0,0 +1,51 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Unit\FormBuilder\Bindings;
+
+use App\FormBuilder\Bindings\BindingApplicationResult;
+use PHPUnit\Framework\TestCase;
+use RuntimeException;
+
+final class BindingApplicationResultTest extends TestCase
+{
+    public function test_succeeded_constructor_populates_state_correctly(): void
+    {
+        $result = BindingApplicationResult::succeeded(
+            bindingId: 'bnd-1',
+            targetEntity: 'person',
+            targetAttribute: 'email',
+            oldValue: null,
+            newValue: 'jan@example.nl',
+        );
+
+        $this->assertTrue($result->success);
+        $this->assertSame('bnd-1', $result->bindingId);
+        $this->assertSame('person', $result->targetEntity);
+        $this->assertSame('email', $result->targetAttribute);
+        $this->assertNull($result->oldValue);
+        $this->assertSame('jan@example.nl', $result->newValue);
+        $this->assertNull($result->exceptionClass);
+        $this->assertNull($result->exceptionMessage);
+    }
+
+    public function test_failed_constructor_captures_exception_metadata(): void
+    {
+        $exception = new RuntimeException('boom');
+
+        $result = BindingApplicationResult::failed(
+            bindingId: 'bnd-2',
+            targetEntity: 'company',
+            targetAttribute: 'name',
+            e: $exception,
+        );
+
+        $this->assertFalse($result->success);
+        $this->assertSame('bnd-2', $result->bindingId);
+        $this->assertNull($result->oldValue);
+        $this->assertNull($result->newValue);
+        $this->assertSame(RuntimeException::class, $result->exceptionClass);
+        $this->assertSame('boom', $result->exceptionMessage);
+    }
+}