[Backend] Establish standalone build infrastructure

Add the thin monorepo CMake entry point and an independently configurable backend project. Enforce the Clang/C++23 toolchain, expose Make/Ninja-neutral tests and developer checks, export an installable RCCBackend package, and introduce the first Support error abstraction.

1 files changed, 49 insertions, 0 deletions

diff --git a/backend/include/rcc/Support/Error.h b/backend/include/rcc/Support/Error.h
new file mode 100644
index 0000000..3d50b46
--- /dev/null
+++ b/backend/include/rcc/Support/Error.h
@@ -0,0 +1,49 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+#ifndef RCC_SUPPORT_ERROR_H
+#define RCC_SUPPORT_ERROR_H
+
+#include <cstdint>
+#include <expected>
+#include <string>
+#include <string_view>
+#include <utility>
+
+namespace rcc {
+
+enum class ErrorCode : std::uint8_t {
+  InvalidArgument,
+  ParseError,
+  VerificationError,
+  Unsupported,
+  IoError,
+  InternalError,
+};
+
+[[nodiscard]] std::string_view getErrorCodeName(ErrorCode Code) noexcept;
+
+class Error final {
+public:
+  Error(ErrorCode Code, std::string Message);
+
+  [[nodiscard]] ErrorCode code() const noexcept {
+    return Code;
+  }
+  [[nodiscard]] std::string_view message() const noexcept {
+    return Message;
+  }
+
+private:
+  ErrorCode Code;
+  std::string Message;
+};
+
+template <typename ValueT> using Expected = std::expected<ValueT, Error>;
+
+[[nodiscard]] inline std::unexpected<Error> makeUnexpected(ErrorCode Code, std::string Message) {
+  return std::unexpected<Error>(Error(Code, std::move(Message)));
+}
+
+} // namespace rcc
+
+#endif // RCC_SUPPORT_ERROR_H