server: cached repeat changes deterministic Llama 3 output #38

Upstream: ggml-org#28527

An identical temperature:0, seed:0 request produces different text after its long prefix is reused from the server prompt cache.

Environment: current main 67672dc5b, Linux aarch64, NVIDIA GB10, CUDA 13.0, Meta-Llama-3-8B-Instruct Q4_0, 3,339-token prompt.

llama-server -m model.gguf -ngl 999 -c 4096 --cache-type-k iq4_nl --no-kv-offload
# Send the same request three times with temperature=0, seed=0, max_tokens=16.

Observed: response 1 differed; responses 2 and 3 matched each other. With --no-cache-prompt, all three responses were identical.

This is not specific to quantized K cache: --cache-type-k f16 also changed after the first response. Cached repeats back off one token and evaluate only that token, while the first request evaluates the full prompt; the two paths produce sufficiently different logits to change greedy decoding.