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/errors.go

@@ -0,0 +1,46 @@
+package maxminddb
+
+import (
+	"fmt"
+	"reflect"
+)
+
+// InvalidDatabaseError is returned when the database contains invalid data
+// and cannot be parsed.
+type InvalidDatabaseError struct {
+	message string
+}
+
+func newOffsetError() InvalidDatabaseError {
+	return InvalidDatabaseError{"unexpected end of database"}
+}
+
+func newInvalidDatabaseError(format string, args ...any) InvalidDatabaseError {
+	return InvalidDatabaseError{fmt.Sprintf(format, args...)}
+}
+
+func (e InvalidDatabaseError) Error() string {
+	return e.message
+}
+
+// UnmarshalTypeError is returned when the value in the database cannot be
+// assigned to the specified data type.
+type UnmarshalTypeError struct {
+	Type  reflect.Type
+	Value string
+}
+
+func newUnmarshalTypeStrError(value string, rType reflect.Type) UnmarshalTypeError {
+	return UnmarshalTypeError{
+		Type:  rType,
+		Value: value,
+	}
+}
+
+func newUnmarshalTypeError(value any, rType reflect.Type) UnmarshalTypeError {
+	return newUnmarshalTypeStrError(fmt.Sprintf("%v (%T)", value, value), rType)
+}
+
+func (e UnmarshalTypeError) Error() string {
+	return fmt.Sprintf("maxminddb: cannot unmarshal %s into type %s", e.Value, e.Type)
+}