KV cache benchmark: KVarN 6-bit quantization beats q8_0 on Qwen 3.6 and Gemma 4

Benchmark of 413 KV cache quantization pairs on Qwen 3.6 27B and Gemma 4 31B finds KVarN 6-bit outperforms q8_0, with the precision tail of 1024 tokens dominating KLD loss. Tests ran on BeeLlama.cpp v0.4.0, a llama.cpp fork with expanded KV cache quantization options.