Typing or pasting a line of 128 characters or more into the CLI hangs the
editor and overruns cmd_buffer[128]: the length check did not reserve room
for the terminating NUL written on Enter, and on a full buffer the character
was retried via continue without advancing the serial-ring read pointer at
the bottom of the loop, so input processing never caught up again.
Cap the line at CMD_BUF_SIZE-1 and, when full, drop the character but fall
through to consume the ring byte instead of spinning on it. The functional
change is three lines; the rest of the diff is re-indentation of the
insert-and-echo block (git diff -w shows the minimal form).
