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.

docs/PHASES.md

@@ -20,10 +20,10 @@ previous one is validated.
 **Goal**: encode raw pixel buffers to H.264 and decode them back, purely with
 FFmpeg software paths.
 
-- Add `libavcodec`, `libavutil`, `libswscale` dependencies.
-- Implement `EncoderFactory::create()` and `Encoder::encode()`.
-- Implement `DecoderFactory::create()` and `Decoder::decode()`.
-- Round-trip test: synthetic RGB frames → H.264 → decoded RGB.
+- [x] Add `libavcodec`, `libavutil`, `libswscale` dependencies.
+- [x] Implement `EncoderFactory::create()` and `Encoder::encode()`.
+- [x] Implement `DecoderFactory::create()` and `Decoder::decode()`.
+- [x] Round-trip test: synthetic RGB frames → H.264 → decoded RGB.
 
 **Validation**: unit test produces visually/structurally correct round-trip
 frames.
@@ -89,4 +89,4 @@ where available.
 
 ## Current phase
 
-Phase 2 — software H.264 encode/decode.
+Phase 3 — PipeWire screen capture.