chore: import upstream snapshot with attribution

Sources/ContainerPlugin/PluginStateRoot.swift

@@ -0,0 +1,36 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2026 Apple Inc. and the container project authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//===----------------------------------------------------------------------===//
+
+import ContainerizationError
+import Foundation
+import SystemPackage
+
+public struct PluginStateRoot {
+    private let plugin: FilePath.Component
+
+    public init(plugin: String) throws {
+        guard let plugin = FilePath.Component(plugin) else {
+            throw ContainerizationError(.invalidArgument, message: "invalid plugin name \(plugin)")
+        }
+        self.plugin = plugin
+    }
+
+    public var path: FilePath {
+        ApplicationRoot.path
+            .appending(FilePath.Component("plugin-state"))
+            .appending(plugin)
+    }
+}