Reads and caches the named column(s) of an open parquet_reader
(reader) right away rather than lazily on first request.
parquet_prefetch_columns accepts names as either an array of column
names (each element sharing one declared length -- pad shorter names
with blanks) or a single scalar string listing the names separated by
commas and/or semicolons ("ra;dec,mag"). The scalar form avoids the
fixed-length array pitfall where a too-short declared length silently
truncates a name.