Refine sender threading and FEC tuning

[abkarada]

Summary

• Limit x264 encoder to two threads and enable appsink signal mode with a new-sample callback for paced packet handling
• Simplify sender main loop to focus on adaptation and stats
• Cap adaptive controller block size at k=16
• Replace lambda-based new-sample handler with a static callback to satisfy GCallback requirements

Testing

• apt-get install -y libgstreamer1.0-dev libgstreamer-plugins-base1.0-dev
• cmake -S . -B build (fails: Cannot find source file /workspace/Full_CPU_Version/third_party/CauchyCaterpillar/CCatCodec.cpp)

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https://chatgpt.com/codex/tasks/task_e_68a873d62690832cab9598a18afb7bf3