Use IExcelDataReader directly by default for lower memory usage.
The AsDataSet extension is available via --dataset (and is implied
by --fill-merged). Raw mode supports all --output formats:
csv/tsv streams row-by-row, table buffers as string arrays.

Table output is capped at 100 rows to prevent hangs on large files;
a note on stderr directs users to --output csv for full data.

In single-pass mode, trailing empty cells may not be present in every
row, leading to varying field counts. RenderRawTable now computes the
true max column count across all buffered rows and pads shorter rows
with empty strings to prevent Spectre.Console from throwing.