cut: refactor multi-byte updates

cut: refactor multi-byte updates

* src/cut.c: 160 fewer lines

Helpers extracted (replacing repeated inline patterns):
- write_line_delim(), write_pending_line_delim(), reset_item_line()
  - line boundary code used by cut_bytes{,no_split}, cut_characters
- write_selected_item()
  - output-delimiter + write logic used by all three byte/char functions
- reset_field_line()
  - field line reset used by cut_fields_mb_any

Field functions unified via cut_fields_mb_any(stream, whitespace_mode):
- struct mbfield_parser encapsulates the whitespace vs.
  fixed-delimiter state (saved char, mode flag)
- mbfield_get_char() - dispatches to saved-char or direct read
- mbfield_terminator()
  - returns FIELD_{DATA,DELIMETER,LINE_DELIMITER} based on mode
- read_mb_field_to_buffer()
  - replaces the two duplicated first-field buffering loops
- scan_mb_field(mbbuf, parser, pending, write_field)
  - replaces the four duplicated field scan loops
  (print+skip × two modes) with a single function and a write_field bool
- cut_fields_mb and cut_fields_ws are now trivial wrappers