feat(audio-cpp): implement runtime lifecycle

Add protocol-neutral model configuration and runtime ownership against the pinned audio.cpp interfaces. Validate task capabilities, preserve the active model on replacement failures, serialize inference calls, and guarantee session-first teardown.

Assisted-by: Codex:gpt-5

backend/cpp/audio-cpp/model_config.h

@@ -0,0 +1,26 @@
+// SPDX-License-Identifier: MIT
+
+#pragma once
+
+#include "engine/framework/runtime/model.h"
+#include "engine/framework/runtime/session.h"
+
+#include <filesystem>
+#include <string>
+#include <unordered_map>
+
+namespace audio_cpp {
+
+struct AudioCppModelConfig {
+    std::filesystem::path model_path;
+    std::string family;
+    engine::runtime::TaskSpec task;
+    engine::runtime::ModelLoadRequest load;
+    engine::runtime::SessionOptions session;
+};
+
+AudioCppModelConfig parse_model_config(
+    const std::filesystem::path & model_path,
+    const std::unordered_map<std::string, std::string> & options);
+
+}  // namespace audio_cpp