Add opt-in physical row indices via rowIndex symbol

[platypii]

Filtered reads compact the returned rows, leaving consumers unable to recover their physical positions in the source file. Add opt-in includeRowIndex: true and an exported rowIndex symbol so callers can read the absolute, zero-based position with row[rowIndex] while retaining filter pushdown.

The property is non-enumerable and read-only, survives filtering, projection, page pruning, and ordered parquetQuery reads, and is omitted by JSON serialization and object spread. The option defaults to false and requires object output. Includes the ParquetRow type and usage documentation.

Validation: all 662 tests pass, including physical positions across ranges, row groups, filtering, projection, page pruning, and ordered queries. ESLint and tsc --noEmit pass.