Shard directory written by regenDrafterData.
Drafter architecture. tapLayers MUST equal the regen's.
Held-out anchors per evaluation.
Cap on the prefix context each anchor attends.
Checkpoint directory (best held-out τ is saved here).
Warm-start parameters from outDir when a checkpoint exists there (the
optimizer and schedule restart; the checkpoint's own config wins).
Identity stamped into dspark.json, e.g. gemma-4-e4b-it-OptiQ-4bit@2560x262144.
Steps a loaded shard serves before the next one is loaded.
Anchors per step.