{
  "sharded": true,
  "shard_count": 4000,
  "shard_key": "pad4(Number(npsn) % SHARD_COUNT)",
  "shard_dir": "by_npsn/shards",
  "shard_file": "by_npsn/shards/<shard>.json",
  "value_shape": "{ \"<npsn>\": <full school record>, ... }",
  "example": {
    "npsn": "20532914",
    "shard": "0914",
    "file": "by_npsn/shards/0914.json"
  }
}