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/error.h

@@ -0,0 +1,36 @@
+#pragma once
+
+#include <string>
+#include <variant>
+
+namespace sc {
+
+struct CodecError {
+    std::string message;
+};
+
+// C++20 does not provide std::expected. Use a variant-based result type so
+// fallible codec operations do not rely on exceptions.
+template <typename T> using CodecResult = std::variant<T, CodecError>;
+
+template <typename T> constexpr bool is_codec_error(const CodecResult<T>& result) noexcept {
+    return std::holds_alternative<CodecError>(result);
+}
+
+template <typename T> T& codec_value(CodecResult<T>& result) {
+    return std::get<T>(result);
+}
+
+template <typename T> const T& codec_value(const CodecResult<T>& result) {
+    return std::get<T>(result);
+}
+
+template <typename T> CodecError& codec_error(CodecResult<T>& result) {
+    return std::get<CodecError>(result);
+}
+
+template <typename T> const CodecError& codec_error(const CodecResult<T>& result) {
+    return std::get<CodecError>(result);
+}
+
+} // namespace sc