feat: VMIN/VTIME termios fields for non-canonical TTY reads

feat: VMIN/VTIME termios fields for non-canonical TTY reads

- Added c_cc[NCCS] array to struct termios with VMIN/VTIME indices
- TCGETS/TCSETS now get/set c_cc values
- tty_read_kbuf implements full POSIX non-canonical semantics:
  VMIN=0,VTIME=0: poll (return immediately)
  VMIN>0,VTIME=0: block until VMIN chars available
  VMIN=0,VTIME>0: timeout read (tenths of second)
  VMIN>0,VTIME>0: block with inter-char timeout
- tty_read refactored to delegate to tty_read_kbuf (no duplication)
- Default: VMIN=1, VTIME=0 (standard blocking single-char read)
- cppcheck clean, 19/19 smoke tests pass