Scaffold C++20 screencast project with Meson, agent workflow, and phase plan

include/screencast/codec/decoder.h

@@ -0,0 +1,39 @@
+#pragma once
+
+#include "screencast/codec/encoder.h"
+
+#include <cstdint>
+#include <memory>
+#include <optional>
+#include <span>
+#include <vector>
+
+namespace sc {
+
+struct DecodedFrame {
+    int width = 0;
+    int height = 0;
+    uint64_t capture_timestamp_ns = 0;
+    std::vector<std::byte> rgba_pixels;
+};
+
+struct DecoderConfig {
+    std::string_view codec_name = "h264";
+    int width = 0;
+    int height = 0;
+};
+
+class Decoder {
+  public:
+    virtual ~Decoder () = default;
+
+    // Feed one encoded frame. Returns decoded frames when available.
+    virtual std::vector<DecodedFrame> decode ( const EncodedFrame &frame ) = 0;
+};
+
+class DecoderFactory {
+  public:
+    static std::unique_ptr<Decoder> create ( const DecoderConfig &config );
+};
+
+}  // namespace sc