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/sqlite/mutex.go

@@ -0,0 +1,23 @@
+// Copyright 2019 The Sqlite 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 sqlite // import "modernc.org/sqlite"
+
+import (
+	"sync"
+	"unsafe"
+
+	"modernc.org/libc"
+	"modernc.org/libc/sys/types"
+)
+
+type mutex struct {
+	sync.Mutex
+}
+
+func mutexAlloc(tls *libc.TLS) uintptr {
+	return libc.Xcalloc(tls, 1, types.Size_t(unsafe.Sizeof(mutex{})))
+}
+
+func mutexFree(tls *libc.TLS, m uintptr) { libc.Xfree(tls, m) }