Delimited text metadata

CSV, TSV, tab-separated, and PSV files all serialize as csv_metadata (delimiter differs by extension).

`csv_metadata`

Flattened column statistics (ColumnarCommonFields) plus dialect fields:

Field	Meaning
`row_count`, `column_count`, `stats_rows_sampled`, `columns`	Table stats and compact `ColumnStat` profiles
`delimiter`, `quote_character`, `escape_character`	Detected CSV dialect
`has_header`	Header row present
`encoding`	Text encoding when detected

The columns[] array uses compact keys (i, t, num, …). See Column statistics for field definitions and typed stat objects.