cpu-amx README: document the bf16 DiT tier (--dit-precision)

cpu-amx: add int8 encoders (same_{s,l}_encoder_int8, SmoothQuant+GPTQ) + README row

cpu-amx README: consistent <model>_<precision> filenames

Add cpu-amx: torch-free C++/AMX engines (T5Gemma bf16, medium DiT int8, SAME-S/L bf16+int8 decoders, SAME-S/L encoders) + weight blobs + README