Add Nix flake for mcproxyctl

Vendor dependencies and expose mcproxyctl binary via nix build.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/google.golang.org/protobuf/protoadapt/convert.go

@@ -0,0 +1,31 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package protoadapt bridges the original and new proto APIs.
+package protoadapt
+
+import (
+	"google.golang.org/protobuf/proto"
+	"google.golang.org/protobuf/runtime/protoiface"
+	"google.golang.org/protobuf/runtime/protoimpl"
+)
+
+// MessageV1 is the original [github.com/golang/protobuf/proto.Message] type.
+type MessageV1 = protoiface.MessageV1
+
+// MessageV2 is the [google.golang.org/protobuf/proto.Message] type used by the
+// current [google.golang.org/protobuf] module, adding support for reflection.
+type MessageV2 = proto.Message
+
+// MessageV1Of converts a v2 message to a v1 message.
+// It returns nil if m is nil.
+func MessageV1Of(m MessageV2) MessageV1 {
+	return protoimpl.X.ProtoMessageV1Of(m)
+}
+
+// MessageV2Of converts a v1 message to a v2 message.
+// It returns nil if m is nil.
+func MessageV2Of(m MessageV1) MessageV2 {
+	return protoimpl.X.ProtoMessageV2Of(m)
+}