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/audio_cpp_runtime.h

@@ -0,0 +1,46 @@
+// SPDX-License-Identifier: MIT
+
+#pragma once
+
+#include "model_config.h"
+
+#include "engine/framework/runtime/registry.h"
+#include "engine/framework/runtime/session.h"
+
+#include <memory>
+#include <mutex>
+
+namespace audio_cpp {
+
+class AudioCppRuntime {
+public:
+    AudioCppRuntime();
+    explicit AudioCppRuntime(engine::runtime::ModelRegistry registry);
+    ~AudioCppRuntime();
+
+    AudioCppRuntime(const AudioCppRuntime &) = delete;
+    AudioCppRuntime & operator=(const AudioCppRuntime &) = delete;
+
+    void load(const AudioCppModelConfig & config);
+    void free();
+
+    engine::runtime::TaskResult run(
+        const engine::runtime::TaskRequest & request);
+    void start_stream(const engine::runtime::TaskRequest & request);
+    engine::runtime::StreamEvent process_audio_chunk(
+        const engine::runtime::AudioChunk & chunk);
+    engine::runtime::TaskResult finish_stream();
+
+private:
+    engine::runtime::IVoiceTaskSession & require_session_locked();
+    engine::runtime::IOfflineVoiceTaskSession & require_offline_locked();
+    engine::runtime::IStreamingVoiceTaskSession & require_streaming_locked();
+    void free_locked();
+
+    std::mutex mutex_;
+    engine::runtime::ModelRegistry registry_;
+    std::unique_ptr<engine::runtime::ILoadedVoiceModel> model_;
+    std::unique_ptr<engine::runtime::IVoiceTaskSession> session_;
+};
+
+}  // namespace audio_cpp