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/internal/impl/enum.go

@@ -0,0 +1,21 @@
+// Copyright 2019 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 impl
+
+import (
+	"reflect"
+
+	"google.golang.org/protobuf/reflect/protoreflect"
+)
+
+type EnumInfo struct {
+	GoReflectType reflect.Type // int32 kind
+	Desc          protoreflect.EnumDescriptor
+}
+
+func (t *EnumInfo) New(n protoreflect.EnumNumber) protoreflect.Enum {
+	return reflect.ValueOf(n).Convert(t.GoReflectType).Interface().(protoreflect.Enum)
+}
+func (t *EnumInfo) Descriptor() protoreflect.EnumDescriptor { return t.Desc }