chore: import upstream snapshot with attribution

Sources/Containerization/Interface.swift

@@ -0,0 +1,46 @@
+//===----------------------------------------------------------------------===//
+// 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.
+//===----------------------------------------------------------------------===//
+
+import ContainerizationExtras
+
+/// A network interface.
+public protocol Interface: Sendable {
+    /// The interface IPv4 address and subnet prefix length, as a CIDR address.
+    /// Example: `192.168.64.3/24`
+    var ipv4Address: CIDRv4 { get }
+
+    /// The IPv4 gateway address for the default route, or nil for no IPv4 default route.
+    var ipv4Gateway: IPv4Address? { get }
+
+    /// The interface IPv6 address and subnet prefix length, as a CIDRv6 address, or nil for no IPv6 address.
+    /// Example: `fd00::1/64`
+    var ipv6Address: CIDRv6? { get }
+
+    /// The IPv6 gateway address for the default route, or nil for no IPv6 default route.
+    var ipv6Gateway: IPv6Address? { get }
+
+    /// The interface MAC address, or nil to auto-configure the address.
+    var macAddress: MACAddress? { get }
+
+    /// The interface MTU (Maximum Transmission Unit).
+    var mtu: UInt32 { get }
+}
+
+extension Interface {
+    public var mtu: UInt32 { 1500 }
+    public var ipv6Address: CIDRv6? { nil }
+    public var ipv6Gateway: IPv6Address? { nil }
+}