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/modernc.org/memory/counters.go

@@ -0,0 +1,10 @@
+// Copyright 2017 The Memory Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build memory.counters
+// +build memory.counters
+
+package memory // import "modernc.org/memory"
+
+const counters = true