learnc tunes C constants, cutting JPEG size 9.1%

An open-source tool, learnc, tunes named constants in unmodified C code against a user-written C scoring function and writes the file back. On held-out data it cut JPEG BD-rate by 9.1% at equal SSIM via stb_image_write.h tables and raised WebRTC VAD F1 from 0.810 to 0.859. It suits tens to hundreds of constants, gives no gain below 1 bpp, and is MIT-licensed.