DataFusion file scans with framework splits

Applies to: the Parquet source scan

What it is

Parquet is now read through DataFusion's file-scan framework — projection pushed into the decode, maintained readers, row-group split granularity — rather than through hand-rolled readers (ff98896).

Measured

Lifted the Parquet copy 4.68x → 4.97x.

Note

ORC rode the same scan core until its source was removed — issue #19.