speculative-simple : add checkpoint support

[ggerganov]

Overview

cont #19493

• Add speculative checkpoints to speculative-simple
• Avoid cloning the sampler in llama-server when not necessary

Additional information

Sample command:

make -j && ./bin/llama-speculative-simple -hf ggml-org/Qwen3.5-35B-A3B-GGUF:Q8_0 -md ~/models/qwen3.5-0.8b-base/ggml-model-q8_0.gguf -p "Here is a quick sort implementation in C++. Just code, no comments:\n\n#include" -n 256 --draft 32 --temp 0 --top-k 1 --seed 42 -ngl 99 -ngld 99

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: YES. pi + Qwen3.6