FdbMappedFile maps a file read-only (CreateFileMapping/MapViewOfFile on
Windows, mmap elsewhere) and falls back to reading it into memory when
mapping fails. FdbReader reads the table and column headers from it and
looks rows up by their first column through the fdb's own buckets,
decoding every integer as little-endian with bounds checks, so the rows
never get copied out of the file.
Tests write small fdb files (collisions, text, int64, nulls) and read
them both mapped and from memory.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
