chore: import upstream snapshot with attribution

Sources/ContainerizationOCI/Manifest.swift

@@ -0,0 +1,57 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2025-2026 Apple Inc. and the Containerization 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.
+//===----------------------------------------------------------------------===//
+
+//  Source: https://github.com/opencontainers/image-spec/blob/main/specs-go/v1/manifest.go
+
+import Foundation
+
+/// Manifest provides `application/vnd.oci.image.manifest.v1+json` mediatype structure when marshalled to JSON.
+public struct Manifest: Codable, Sendable {
+    /// `schemaVersion` is the image manifest schema that this image follows.
+    public let schemaVersion: Int
+
+    /// `mediaType` specifies the type of this document data structure, e.g. `application/vnd.oci.image.manifest.v1+json`.
+    public let mediaType: String?
+
+    /// `config` references a configuration object for a container, by digest.
+    /// The referenced configuration object is a JSON blob that the runtime uses to set up the container.
+    public let config: Descriptor
+
+    /// `layers` is an indexed list of layers referenced by the manifest.
+    public let layers: [Descriptor]
+
+    /// `annotations` contains arbitrary metadata for the image manifest.
+    public let annotations: [String: String]?
+
+    /// `subject` references another manifest this manifest is an artifact of.
+    public let subject: Descriptor?
+
+    /// `artifactType` specifies the IANA media type of the artifact this manifest represents.
+    public let artifactType: String?
+
+    public init(
+        schemaVersion: Int = 2, mediaType: String = MediaTypes.imageManifest, config: Descriptor, layers: [Descriptor],
+        annotations: [String: String]? = nil, subject: Descriptor? = nil, artifactType: String? = nil
+    ) {
+        self.schemaVersion = schemaVersion
+        self.mediaType = mediaType
+        self.config = config
+        self.layers = layers
+        self.annotations = annotations
+        self.subject = subject
+        self.artifactType = artifactType
+    }
+}