Replace LineIndex::from_reader(BufReader) with LineIndex::from_bytes(&mmap)
in both open() and reload(), ensuring the line index is always built from
the same mmap snapshot rather than a separate read through the file descriptor.
This closes the race window where an external file modification between
mmap() and from_reader() could cause line_index offsets to disagree with
the mmap data, leading to get_line() returning wrong content or panicking.
Closes#2
24fe97a457