Adds a plain stderr log handler. (#680)

- Initial work for #642. CLI commands will configure this log handler so
that they write warnings and errors to stderr, and only proper output
will go to stdout.
- Facilitates #507. The `--debug` option can set the logging level
appropriately, and CLI commands can just use `log.debug()`.

## Type of Change
- [ ] Bug fix
- [x] New feature  
- [ ] Breaking change
- [ ] Documentation update

Sources/ContainerLog/StderrLogHandler.swift

@@ -0,0 +1,56 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2025 Apple Inc. and the container project authors. All rights reserved.
+//
+// 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 Foundation
+import Logging
+
+/// Basic log handler for where simple message output is needed,
+/// such as CLI commands.
+public struct StderrLogHandler: LogHandler {
+    public var logLevel: Logger.Level = .info
+    public var metadata: Logger.Metadata = [:]
+
+    public subscript(metadataKey metadataKey: String) -> Logger.Metadata.Value? {
+        get {
+            self.metadata[metadataKey]
+        }
+        set {
+            self.metadata[metadataKey] = newValue
+        }
+    }
+
+    public init() {}
+
+    public func log(
+        level: Logger.Level,
+        message: Logger.Message,
+        metadata: Logger.Metadata?,
+        source: String,
+        file: String,
+        function: String,
+        line: UInt
+    ) {
+        let messageText = message.description
+        let data = messageText.data(using: .utf8) ?? Data()
+
+        // Use a single write call for atomicity
+        var output = data
+        output.append("\n".data(using: .utf8)!)
+        FileHandle.standardError.write(output)
+    }
+}