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/libc/tls_linux_amd64.go

@@ -0,0 +1,33 @@
+// Copyright 2025 The Libc 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 libc // import "modernc.org/libc"
+
+//go:noescape
+func TLSAlloc(p0 *TLS, p1 int) uintptr
+
+//go:noescape
+func TLSFree(p0 *TLS, p1 int)
+
+//go:noescape
+func TLSAllocaEntry(p0 *TLS)
+
+//go:noescape
+func TLSAllocaExit(p0 *TLS)
+
+func tlsAlloc(tls *TLS, n int) uintptr {
+	return tls.Alloc(n)
+}
+
+func tlsFree(tls *TLS, n int) {
+	tls.Free(n)
+}
+
+func tlsAllocaEntry(tls *TLS) {
+	tls.AllocaEntry()
+}
+
+func tlsAllocaExit(tls *TLS) {
+	tls.AllocaExit()
+}