paste -s -d does not reset the delimiter list per file (diverges from GNU) #13438

Bug

In paste -s (serial mode) with multiple delimiters (-d/--delimiters), the delimiter list is not restarted for each file. Each input file becomes its own output line, so the delimiter list should restart from its first element for each file — as GNU does, and as uutils's own non-serial branch already does — but the DelimiterState cycle is created once and carried across files.

Reproduce

$ printf 'a\nb\n' > f1; printf 'c\nd\ne\n' > f2; printf 'f\ng\n' > f3
$ paste -s -d ':|' f1 f2 f3

Another example: paste -s -d '123' f1 f2 → uutils .../13213, GNU .../11223.

Cause / fix

DelimiterState::reset_to_first_delimiter() already exists and is called per output line in the non-serial branch, but the serial branch never calls it between files. Calling it at the end of each file iteration in the serial loop makes paste -s -d match GNU. Verified against GNU coreutils across a delimiter/file matrix. PR incoming.