Type Alias Column

The framework's value-column discriminated union. Narrow on column.kind first, then on column.storage to recover the concrete shape:

if (col.kind === 'number') {
  if (col.storage === 'packed') {
    col._values; // Float64Array (Float64Column)
  } else {
    col.chunks; // ReadonlyArray<Float64Column> (ChunkedFloat64Column)
  }
}

All four kinds ('number', 'boolean', 'string', 'array') are concrete classes after sub-step 1c. Sub-step 1g adds chunked counterparts for each.