Lets the vocab maps, event-id split index, and normalizer stats be
warmed once for a dataset (right after `dwarf convert`, or before a
`dwarf hparam-scan` sweep) without needing to also start training.
Extracts the setup-stage logic out of giant/pipeline.py:run_train_job
into a standalone run_setup_stage() (returning a SetupStageResult),
reused by both run_train_job and the new dwarf command's
scripts/warm_setup_cache.py — a behavior-preserving refactor, covered
by the existing test_pipeline.py suite.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
