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/app/FormBuilder/Bindings/BindingApplicationResult.php

@@ -0,0 +1,61 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\FormBuilder\Bindings;
+
+use Throwable;
+
+/**
+ * RFC-WS-6 §3 (Q4) — the result of applying a single resolved binding
+ * to its target. Sealed via two named constructors so consumers can't
+ * synthesise impossible states (success-with-exception, etc.).
+ */
+final readonly class BindingApplicationResult
+{
+    private function __construct(
+        public string $bindingId,
+        public string $targetEntity,
+        public string $targetAttribute,
+        public bool $success,
+        public mixed $oldValue,
+        public mixed $newValue,
+        public ?string $exceptionClass = null,
+        public ?string $exceptionMessage = null,
+    ) {}
+
+    public static function succeeded(
+        string $bindingId,
+        string $targetEntity,
+        string $targetAttribute,
+        mixed $oldValue,
+        mixed $newValue,
+    ): self {
+        return new self(
+            bindingId: $bindingId,
+            targetEntity: $targetEntity,
+            targetAttribute: $targetAttribute,
+            success: true,
+            oldValue: $oldValue,
+            newValue: $newValue,
+        );
+    }
+
+    public static function failed(
+        string $bindingId,
+        string $targetEntity,
+        string $targetAttribute,
+        Throwable $e,
+    ): self {
+        return new self(
+            bindingId: $bindingId,
+            targetEntity: $targetEntity,
+            targetAttribute: $targetAttribute,
+            success: false,
+            oldValue: null,
+            newValue: null,
+            exceptionClass: $e::class,
+            exceptionMessage: $e->getMessage(),
+        );
+    }
+}