fix: isolate dataset shuffle random state

[LZC-BELIEVER]

Summary

• Use a dataset-local random.Random instance for deterministic prompt shuffling.
• Preserve the existing seed + epoch permutation behavior without mutating Python's process-global RNG state.
• Add CPU regression tests for permutation compatibility, reproducibility, and RNG isolation.
• Register the new test in the regular CPU CI matrix.

Problem

Dataset.shuffle() currently reseeds Python's module-level random number generator whenever the dataset changes epoch:

random.seed(self.seed + new_epoch_id)
random.shuffle(permutation)

RolloutDataSource invokes this method during initialization and at epoch boundaries. As a result, shuffling the prompt dataset also resets the random stream observed by unrelated code in the same long-lived rollout process, including custom generation or reward functions that use Python's random module.

This creates hidden coupling between dataset ordering and otherwise independent random operations. Adding or moving a shuffle call can therefore change behavior outside the dataset.

Fix

Create a dedicated RNG for each deterministic shuffle:

rng = random.Random(self.seed + new_epoch_id)
rng.shuffle(permutation)

The local RNG uses the same seed and standard-library shuffle implementation as before, so the dataset permutation remains deterministic for each epoch. Its state is confined to Dataset.shuffle(), leaving the process-global RNG unchanged.

No command-line arguments, dataset APIs, sampling parameters, or GPU code paths are changed.

Testing

The new CPU tests verify that:

• the seeded permutation is preserved;
• Dataset.shuffle() does not modify the process-global RNG state;
• independent datasets produce the same permutation for the same seed and epoch.

Relevant CPU regression suite:

PYTHONPATH=. python -m pytest \
  tests/test_dataset_shuffle.py \
  tests/test_filter_long_prompt.py \
  tests/test_read_file_slicing.py \
  tests/test_docs_consistency.py

Result: 22 passed.

Repository checks:

pre-commit run --files \
  slime/utils/data.py \
  tests/test_dataset_shuffle.py \
  .github/workflows/pr-test.yml.j2 \
  .github/workflows/pr-test.yml \
  --show-diff-on-failure --color=never

git diff --check

All pre-commit hooks passed, and git diff --check reported no errors.