AVS3 Encoder integrate

[xeonliu]

Add AVS3 Encode Support for n7.1.3

This PR adds an FFmpeg encoder wrapper for libuavs3e, an external library that encodes AVS3 (Audio Video Standard 3, China's counterpart to H.266/VVC).
After this change, users can encode AVS3 video with:

ffmpeg -i input.mp4 \
  -c:v libuavs3e -qp 34 \
  -speed 6 output.avs3

The PR has three parts:

• Encoder wrapper — a new file libuavs3e.c:1-295 that bridges FFmpeg's AVCodec interface to the uavs3e library
• Build system wiring — changes to configure, Makefile, and allcodecs.c so FFmpeg can discover and compile the encoder
• AVS3 FourCC registration — a one-line addition to riff.c:378 enabling AVS3 in AVI/RIFF containers