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/Enums/FormBuilder/FormFieldBindingMergeStrategy.php

@@ -10,4 +10,31 @@ enum FormFieldBindingMergeStrategy: string
     case Append = 'append';
     case Replace = 'replace';
     case FirstWriteWins = 'first_write_wins';
+
+    /**
+     * RFC-WS-6 §3 (Q7) — what to do when the conflict-resolution winner
+     * has a null value. Returns one of:
+     *   - 'write'        write null to the target (intent: clear)
+     *   - 'noop'         leave the target untouched
+     *   - 'conditional'  write only when the target itself is null
+     */
+    public function nullWinnerBehaviour(): string
+    {
+        return match ($this) {
+            self::Overwrite => 'write',
+            self::Append => 'noop',
+            self::Replace => 'noop',
+            self::FirstWriteWins => 'conditional',
+        };
+    }
+
+    /**
+     * RFC-WS-6 §4 (V1) — Append is collection-only (idempotent retry only
+     * with set semantics; scalar-append demands fingerprinting which is
+     * an architectural smell).
+     */
+    public function isValidForScalarTargets(): bool
+    {
+        return $this !== self::Append;
+    }
 }