hpb: Introduce HPB_INTERNAL_BACKEND and backend/cpp/cpp.h

This abrogates HPB_BACKEND_UPB and HPB_BACKEND_CPP. The backend is now controlled by a string_flag, which defaults to upb.

Note that INTERNAL was also added -- clarifying that this macro is for internal use only.

This continues the multibackend work by stubbing the second backend in cpp.h.

PiperOrigin-RevId: 750172782

hpb/backend/cpp/cpp.h

diff --git a/hpb/backend/cpp/cpp.h b/hpb/backend/cpp/cpp.h
new file mode 100644
index 0000000..4a0fa53
--- /dev/null
+++ b/hpb/backend/cpp/cpp.h
@@ -0,0 +1,32 @@
+// Protocol Buffers - Google's data interchange format
+// Copyright 2025 Google LLC.  All rights reserved.
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file or at
+// https://developers.google.com/open-source/licenses/bsd
+
+#ifndef GOOGLE_PROTOBUF_HPB_BACKEND_CPP_CPP_H__
+#define GOOGLE_PROTOBUF_HPB_BACKEND_CPP_CPP_H__
+
+#include "absl/strings/string_view.h"
+#include "google/protobuf/hpb/arena.h"
+#include "google/protobuf/hpb/internal/template_help.h"
+
+namespace hpb::internal::backend::cpp {
+
+// hpb(cpp) backend stubs.
+
+template <typename T>
+void ClearMessage(hpb::internal::PtrOrRawMutable<T> message) {
+  abort();
+}
+
+template <typename T>
+absl::string_view Serialize(hpb::internal::PtrOrRaw<T> message,
+                            hpb::Arena& arena) {
+  abort();
+}
+
+}  // namespace hpb::internal::backend::cpp
+
+#endif  // GOOGLE_PROTOBUF_HPB_BACKEND_CPP_CPP_H__

hpb/hpb.h

diff --git a/hpb/hpb.h b/hpb/hpb.h
index 929e347..d47cd01 100644
--- a/hpb/hpb.h
+++ b/hpb/hpb.h
@@ -23,16 +23,25 @@
 #include "google/protobuf/hpb/status.h"
 #include "upb/wire/decode.h"
 
-#ifdef HPB_BACKEND_UPB
+#define HPB_INTERNAL_BACKEND_UPB 1
+#define HPB_INTERNAL_BACKEND_CPP 2
+
+#if HPB_INTERNAL_BACKEND == HPB_INTERNAL_BACKEND_UPB
 #include "google/protobuf/hpb/backend/upb/upb.h"
+#elif HPB_INTERNAL_BACKEND == HPB_INTERNAL_BACKEND_CPP
+#include "google/protobuf/hpb/backend/cpp/cpp.h"
 #else
-#error hpb backend must be specified
+#error hpb backend unknown
 #endif
 
 namespace hpb {
 
-#ifdef HPB_BACKEND_UPB
+#if HPB_INTERNAL_BACKEND == HPB_INTERNAL_BACKEND_UPB
 namespace backend = internal::backend::upb;
+#elif HPB_INTERNAL_BACKEND == HPB_INTERNAL_BACKEND_CPP
+namespace backend = internal::backend::cpp;
+#else
+#error hpb backend unknown
 #endif
 
 template <typename T>