Taking argmax over the n_sec classifier logits collapses secondary
multiplicity onto its conditional mode at fixed pre-step conditioning,
under-dispersing n_sec in rollouts and biasing low wherever the true
conditional count distribution is right-skewed (typical for
multiplicity).
Generalizes stage2_model.n_sec.stop_sampling (previously stop_token-only)
into stage2_model.n_sec.sampling, covering both "head" (greedy: argmax;
sample: categorical draw via torch.multinomial) and "stop_token" (unchanged:
greedy threshold / Bernoulli draw) modes. stop_sampling is kept as a
deprecated alias in NSecConfig.from_dict and migrate_config, since it
appears in existing checkpoints' model_config. Default stays "greedy" so
existing runs/checkpoints are unaffected.
lars