chore: import upstream snapshot with attribution

Sources/Containerization/CHInstanceExtension.swift

@@ -0,0 +1,46 @@
+//===----------------------------------------------------------------------===//
+// 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.
+//===----------------------------------------------------------------------===//
+
+#if os(Linux)
+import CloudHypervisor
+
+/// Extension hook for `CHVirtualMachineInstance` lifecycle. Append conforming
+/// types to `Configuration.extensions` to participate in VM setup and
+/// teardown without subclassing.
+///
+/// All methods have no-op defaults so a conforming type only needs to
+/// implement the hooks it actually cares about.
+public protocol CHInstanceExtension: Sendable {
+    /// Mutate the cloud-hypervisor `VmConfig` before the VM is created.
+    /// Called by `start()` after the base config is built but before
+    /// `vm.create` is dispatched to the VMM.
+    func configureCH(_ config: inout CloudHypervisor.VmConfig) throws
+
+    /// Called once the VM has been created and booted but before
+    /// `start()` returns to the caller.
+    func didCreate(_ instance: CHVirtualMachineInstance) throws
+
+    /// Called from `stop()` before the VM is shut down. Errors are
+    /// best-effort — `stop()` swallows them.
+    func willStop(_ instance: CHVirtualMachineInstance) async throws
+}
+
+extension CHInstanceExtension {
+    public func configureCH(_ config: inout CloudHypervisor.VmConfig) throws {}
+    public func didCreate(_ instance: CHVirtualMachineInstance) throws {}
+    public func willStop(_ instance: CHVirtualMachineInstance) async throws {}
+}
+#endif