保留的采样后缀可以在不同的因果前缀下携带 logprobs

Native multi-history tokenization can mark a retained response suffix `SAMPLED|EXACT` and select it for training after earlier sampled tokens have been removed from its conditioning context. Its stored logprob still belongs to the original longer prefix. This is independent of the terminal renderer-tail work in #882 and does not require the experimental raw-capture adapter.