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_mmap.go

@@ -0,0 +1,64 @@
+//go:build !appengine && !plan9 && !js && !wasip1 && !wasi
+// +build !appengine,!plan9,!js,!wasip1,!wasi
+
+package maxminddb
+
+import (
+	"os"
+	"runtime"
+)
+
+// 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) {
+	mapFile, err := os.Open(file)
+	if err != nil {
+		_ = mapFile.Close()
+		return nil, err
+	}
+
+	stats, err := mapFile.Stat()
+	if err != nil {
+		_ = mapFile.Close()
+		return nil, err
+	}
+
+	fileSize := int(stats.Size())
+	mmap, err := mmap(int(mapFile.Fd()), fileSize)
+	if err != nil {
+		_ = mapFile.Close()
+		return nil, err
+	}
+
+	if err := mapFile.Close(); err != nil {
+		//nolint:errcheck // we prefer to return the original error
+		munmap(mmap)
+		return nil, err
+	}
+
+	reader, err := FromBytes(mmap)
+	if err != nil {
+		//nolint:errcheck // we prefer to return the original error
+		munmap(mmap)
+		return nil, err
+	}
+
+	reader.hasMappedFile = true
+	runtime.SetFinalizer(reader, (*Reader).Close)
+	return reader, nil
+}
+
+// Close returns the resources used by the database to the system.
+func (r *Reader) Close() error {
+	var err error
+	if r.hasMappedFile {
+		runtime.SetFinalizer(r, nil)
+		r.hasMappedFile = false
+		err = munmap(r.buffer)
+	}
+	r.buffer = nil
+	return err
+}