Added ./demo shell script that sets raw mode via stty before running
the Lisp demo and restores it on exit (including SIGINT/SIGTERM).
demo.lisp no longer attempts to set raw mode from inside SBCL —
terminal raw mode is the shell's responsibility. This avoids the
recurring problem of sb-ext:run-program + stty not being able to
access the controlling terminal from inside sbcl --script.
