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.

README.md

@@ -7,7 +7,8 @@ A native Linux peer-to-peer screencast application.
 - Built with **C++20**, **Meson**, **PipeWire**, **FFmpeg**, **RTP/UDP**, and
   **SDL**.
 
-> This project is in early development. Only the skeleton exists so far.
+> This project is in early development. The H.264 codec path is implemented;
+> capture, transport, and rendering are still in progress.
 
 ## Quick start
 
@@ -16,7 +17,8 @@ Requirements:
 - C++20 compiler (GCC 12+, Clang 16+)
 - Meson >= 0.63
 - Ninja
-- (later phases) FFmpeg dev packages, PipeWire dev, SDL2/3 dev
+- FFmpeg development packages (`libavcodec`, `libavutil`, `libswscale`)
+- (later phases) PipeWire dev, SDL2/3 dev
 
 Build and run tests:
 
@@ -40,7 +42,7 @@ See `docs/ARCHITECTURE.md` for module boundaries and design rules.
 
 Development is split into phases in `docs/PHASES.md`.
 
-Current phase: **Phase 2 — software H.264 encode / decode**.
+Current phase: **Phase 3 — PipeWire screen capture**.
 
 ## License