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/github.com/oschwald/maxminddb-golang/reader_memory.go

@@ -0,0 +1,26 @@
+//go:build appengine || plan9 || js || wasip1 || wasi
+// +build appengine plan9 js wasip1 wasi
+
+package maxminddb
+
+import "io/ioutil"
+
+// Open takes a string path to a MaxMind DB file and returns a Reader
+// structure or an error. The database file is opened using a memory map
+// on supported platforms. On platforms without memory map support, such
+// as WebAssembly or Google App Engine, the database is loaded into memory.
+// Use the Close method on the Reader object to return the resources to the system.
+func Open(file string) (*Reader, error) {
+	bytes, err := ioutil.ReadFile(file)
+	if err != nil {
+		return nil, err
+	}
+
+	return FromBytes(bytes)
+}
+
+// Close returns the resources used by the database to the system.
+func (r *Reader) Close() error {
+	r.buffer = nil
+	return nil
+}