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/protolazy/pointer_unsafe.go

@@ -0,0 +1,17 @@
+// Copyright 2024 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 protolazy
+
+import (
+	"sync/atomic"
+	"unsafe"
+)
+
+func atomicLoadIndex(p **[]IndexEntry) *[]IndexEntry {
+	return (*[]IndexEntry)(atomic.LoadPointer((*unsafe.Pointer)(unsafe.Pointer(p))))
+}
+func atomicStoreIndex(p **[]IndexEntry, v *[]IndexEntry) {
+	atomic.StorePointer((*unsafe.Pointer)(unsafe.Pointer(p)), unsafe.Pointer(v))
+}