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.

tests/meson.build

@@ -3,3 +3,9 @@ test_clock = executable('test_clock',
     include_directories : sc_core_inc)
 
 test('clock utils', test_clock)
+
+test_codec_roundtrip = executable('test_codec_roundtrip',
+    'codec/test_roundtrip.cpp',
+    dependencies : sc_codec_dep)
+
+test('h264 roundtrip', test_codec_roundtrip)