feat(codec): implement software H.264 encode/decode round-trip

Add FFmpeg-based encoder/decoder with SPS/PPS extradata, Annex-B output

normalization, low-latency libx264 settings, and a round-trip unit test.

Includes review hardening: cached SwsContext, bitrate-only rate control,

std::byte/uin8_t cast helpers, and richer test assertions.

include/screencast/codec/decoder.h

@@ -1,11 +1,11 @@
 #pragma once
 
 #include "screencast/codec/encoder.h"
+#include "screencast/codec/error.h"
 
 #include <cstdint>
 #include <memory>
-#include <optional>
-#include <span>
+#include <string>
 #include <vector>
 
 namespace sc {
@@ -18,22 +18,23 @@ struct DecodedFrame {
 };
 
 struct DecoderConfig {
-    std::string_view codec_name = "h264";
+    std::string codec_name = "h264";
     int width = 0;
     int height = 0;
+    std::vector<std::byte> extradata; // SPS/PPS for H.264
 };
 
 class Decoder {
   public:
-    virtual ~Decoder () = default;
+    virtual ~Decoder() = default;
 
     // Feed one encoded frame. Returns decoded frames when available.
-    virtual std::vector<DecodedFrame> decode ( const EncodedFrame &frame ) = 0;
+    virtual CodecResult<std::vector<DecodedFrame>> decode(const EncodedFrame& frame) = 0;
 };
 
 class DecoderFactory {
   public:
-    static std::unique_ptr<Decoder> create ( const DecoderConfig &config );
+    static CodecResult<std::unique_ptr<Decoder>> create(const DecoderConfig& config);
 };
 
-}  // namespace sc
+} // namespace sc