Add Nix flake for mciasctl and mciasgrpcctl

Vendor dependencies and expose control program binaries via
nix build. Uses nixpkgs-unstable for Go 1.26 support.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/github.com/go-webauthn/webauthn/protocol/decoder.go

@@ -0,0 +1,40 @@
+package protocol
+
+import (
+	"bytes"
+	"encoding/json"
+	"errors"
+	"io"
+)
+
+func decodeBody(body io.Reader, v any) (err error) {
+	decoder := json.NewDecoder(body)
+
+	if err = decoder.Decode(v); err != nil {
+		return err
+	}
+
+	_, err = decoder.Token()
+
+	if !errors.Is(err, io.EOF) {
+		return errors.New("body contains trailing data")
+	}
+
+	return nil
+}
+
+func decodeBytes(data []byte, v any) (err error) {
+	decoder := json.NewDecoder(bytes.NewReader(data))
+
+	if err = decoder.Decode(v); err != nil {
+		return err
+	}
+
+	_, err = decoder.Token()
+
+	if !errors.Is(err, io.EOF) {
+		return errors.New("body contains trailing data")
+	}
+
+	return nil
+}