fix(ds4): propagate sampler config in layer split

[Graffioh]

Summary

• propagate request sampler settings into the DeepSeek4 layer-split adapter
• reseed the layer-split sampler RNG when sampling with an explicit seed
• add unit coverage for sampler propagation and deterministic seeding

Scope

This PR covers the layer-split path. #527 remains focused on single-device decode correctness.

Known limitation

run_layer_split_ar_decode() currently supplies generated tokens to sample_logits(), but not the prompt or restored-prefix history. Temperature, top-k, and top-p propagation are covered here; repetition, frequency, and presence penalties remain incomplete until both decode paths share a proper request-history contract.

Validation

• git diff --check passed
• unit coverage verifies sampler propagation and deterministic seeding
• runtime GPU validation pending

[Graffioh]

Superseded by #531. The sampler propagation change from this PR has been integrated into #531, where it is combined with the DeepSeek 4 GPU argmax work, mutually exclusive logits/argmax output paths, and CPU/CUDA/HIP parity coverage. Closing this PR so the combined change can be reviewed and merged in one place.