chore: import upstream snapshot with attribution

Tests/ContainerizationTests/InterfaceTests.swift

@@ -0,0 +1,59 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 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
+import Testing
+
+@testable import Containerization
+
+struct InterfaceTests {
+
+    /// A minimal `Interface` conformer that only sets the IPv4 surface, relying on the
+    /// protocol's default extensions to fill in `ipv6Address`, `ipv6Gateway`, and `mtu`.
+    private struct V4OnlyInterface: Interface {
+        let ipv4Address: CIDRv4
+        let ipv4Gateway: IPv4Address?
+        let macAddress: MACAddress?
+    }
+
+    @Test func interfaceProtocolV6Defaults() throws {
+        let i = V4OnlyInterface(
+            ipv4Address: try CIDRv4("10.0.0.2/24"),
+            ipv4Gateway: try IPv4Address("10.0.0.1"),
+            macAddress: nil)
+        #expect(i.ipv6Address == nil)
+        #expect(i.ipv6Gateway == nil)
+        #expect(i.mtu == 1500)
+    }
+
+    @Test func natInterfaceRoundTripsV6Fields() throws {
+        let nat = NATInterface(
+            ipv4Address: try CIDRv4("10.0.0.2/24"),
+            ipv4Gateway: try IPv4Address("10.0.0.1"),
+            ipv6Address: try CIDRv6("fd00::2/64"),
+            ipv6Gateway: try IPv6Address("fd00::1"))
+        #expect(nat.ipv6Address == (try CIDRv6("fd00::2/64")))
+        #expect(nat.ipv6Gateway == (try IPv6Address("fd00::1")))
+    }
+
+    @Test func natInterfaceV6FieldsDefaultToNil() throws {
+        let nat = NATInterface(
+            ipv4Address: try CIDRv4("10.0.0.2/24"),
+            ipv4Gateway: try IPv4Address("10.0.0.1"))
+        #expect(nat.ipv6Address == nil)
+        #expect(nat.ipv6Gateway == nil)
+    }
+}