[MannXo]

In column mode fold dropped zero-width characters (U+200B, joiners, combining marks, a BOM) that came directly after a newline. The loop that attaches zero-width characters to the preceding character ran before the '\n' check, so it consumed them, and the newline branch then emitted the line and continued without pushing those bytes.

The newline check now runs before that loop, so the zero-width characters are handled with the next line as usual.

$ printf 'abc\n\xe2\x80\x8bFrom x\n' | fold | od -c
0000000   a   b   c  \n 342 200 213   F   r   o   m       x  \n

Before, 342 200 213 was missing. -b and -c were not affected.

test_zero_width_after_newline_in_column_mode fails on main.

Closes #15750