Update progress at checkpoint-875

Upload checkpoint-875 (shard 3, 0:506)

Reset next_start_index to 0 after all shards completed (for potential restart)

Update progress at checkpoint-625

Upload checkpoint-625 (shard 2, 0:506)

Reset next_start_index to 0 after all shards completed (for potential restart)

Update progress at checkpoint-375

Upload checkpoint-375 (shard 1, 500:506)

Update progress at checkpoint-125

Upload checkpoint-125 (shard 0, 0:500)

Initial/reset progress.json for new training run

initial commit