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/BindingPassResult.php

@@ -0,0 +1,66 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\FormBuilder\Bindings;
+
+use App\Enums\FormBuilder\ApplyStatus;
+
+/**
+ * RFC-WS-6 §3 (Q4) — aggregate result of one
+ * {@see \App\Services\FormBuilder\FormBindingApplicator} pass over a
+ * submission. `applyStatus()` is the canonical mapping consumed by
+ * `form_submissions.apply_status`.
+ */
+final readonly class BindingPassResult
+{
+    /**
+     * @param  list<BindingApplicationResult>  $applications
+     */
+    public function __construct(
+        public string $formSubmissionId,
+        public ?string $provisionedSubjectType,
+        public ?string $provisionedSubjectId,
+        public array $applications,
+    ) {}
+
+    public function applyStatus(): ApplyStatus
+    {
+        if ($this->applications === []) {
+            return ApplyStatus::COMPLETED;
+        }
+
+        $successes = $this->successCount();
+        $failures = $this->failureCount();
+
+        if ($failures === 0) {
+            return ApplyStatus::COMPLETED;
+        }
+        if ($successes === 0) {
+            return ApplyStatus::FAILED;
+        }
+
+        return ApplyStatus::PARTIAL;
+    }
+
+    public function successCount(): int
+    {
+        return count(array_filter($this->applications, fn (BindingApplicationResult $r): bool => $r->success));
+    }
+
+    public function failureCount(): int
+    {
+        return count(array_filter($this->applications, fn (BindingApplicationResult $r): bool => ! $r->success));
+    }
+
+    /**
+     * @return list<BindingApplicationResult>
+     */
+    public function failures(): array
+    {
+        return array_values(array_filter(
+            $this->applications,
+            fn (BindingApplicationResult $r): bool => ! $r->success,
+        ));
+    }
+}