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/lib/defs.go

@@ -0,0 +1,16 @@
+// Copyright 2022 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 sqlite3
+
+const (
+	SQLITE_STATIC    = uintptr(0)  // ((sqlite3_destructor_type)0)
+	SQLITE_TRANSIENT = ^uintptr(0) // ((sqlite3_destructor_type)-1)
+)
+
+type (
+	Sqlite3_index_constraint       = sqlite3_index_constraint
+	Sqlite3_index_orderby          = sqlite3_index_orderby
+	Sqlite3_index_constraint_usage = sqlite3_index_constraint_usage
+)