feat: add native local-file playback

Sources/Core/output.hpp

@@ -0,0 +1,35 @@
+#pragma once
+#include <cstddef>
+#include <memory>
+#include <string>
+extern "C" {
+#include <libavutil/frame.h>
+}
+
+namespace playback {
+// One decoding worker produces output. The native audio callback is the consumer.
+// Audio is interleaved stereo float at 48 kHz; write() never blocks.
+class Output {
+public:
+    Output();
+    ~Output();
+    Output(const Output &) = delete;
+    Output &operator=(const Output &) = delete;
+    bool configure_audio(std::string &error);
+    bool start(std::string &error);
+    void pause();
+    // Called with audio stopped. Clears audio and video from the old timeline.
+    void reset();
+    std::size_t write(const float *samples, std::size_t frames, double pts);
+    std::size_t queued() const;
+    // Latency-aware presented position; NaN until an audio callback has run.
+    double clock() const;
+    // Retains the frame's buffers; decoder may reuse its AVFrame after return.
+    void present(const AVFrame *frame);
+    // Returns a retained NSView. The view remains safe after Output is destroyed.
+    void *make_video_view();
+private:
+    struct Impl;
+    std::unique_ptr<Impl> impl;
+};
+}