feat(form-builder): validForTargetType method on FormFieldBindingMergeStrategy

Per RFC-WS-6 §V1 + ARCH-BINDINGS §4.2.

Implements the strategy x target-type validity matrix. Append is the
only non-trivial case: valid only for COLLECTION targets. The
AppendStrategyRequiresCollectionTarget publish-guard uses this method
(D2 wiring confirms call sites; this commit provides the building block).

Existing methods (nullWinnerBehaviour, isValidForScalarTargets) untouched.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

api/app/Enums/FormBuilder/FormFieldBindingMergeStrategy.php

@@ -37,4 +37,31 @@ enum FormFieldBindingMergeStrategy: string
     {
         return $this !== self::Append;
     }
+
+    /**
+     * Whether this strategy is structurally valid against the given target
+     * type.
+     *
+     * Per RFC-WS-6 §V1 + ARCH-BINDINGS §4.2 (strategy x target-type validity matrix).
+     *
+     *                  | SCALAR | COLLECTION | RELATION |
+     *  Overwrite       | valid  | valid*     | valid    |
+     *  Append          | INVALID| valid      | INVALID  |
+     *  Replace         | valid  | valid      | valid    |
+     *  FirstWriteWins  | valid  | valid      | valid    |
+     *
+     *  * unusual but valid (overwrites entire collection)
+     *
+     * The PublishGuard AppendStrategyRequiresCollectionTarget uses this
+     * method to validate at publish time. Append on scalars is rejected
+     * because it requires a fingerprint mechanism for retry-idempotency
+     * that would embed implementation detail in domain data.
+     */
+    public function validForTargetType(BindingTargetType $type): bool
+    {
+        return match ($this) {
+            self::Append => $type === BindingTargetType::COLLECTION,
+            default => true,
+        };
+    }
 }