Refine collect summary and training next step

[Tingliangstu]

Summary

• Show all collection source groups when there are 20 or fewer groups, with a source total at the end.
• Reword the van der Waals correction line in user-facing terms rather than internal virial-block layout wording.
• Add a pesmaker train train.yaml command that reuses the smart workflow driver for training YAML files, and update collect/next docs to point to it.

Tests

• python -m ruff check src tests
• pytest tests/test_workflow_stages.py::test_collect_writes_concise_summary_grouped_by_source tests/test_workflow_next.py::test_next_runs_collect_only_config tests/test_cli_generate.py::test_cli_train_command_uses_next_workflow -q
• pytest -q