Bench(fix[scripts]): Mark the executable demo executable

why: The script carries a `uv run --script` shebang but shipped without the
executable bit, so the shebang was misleading: the one invocation it
advertises could not work. ruff's EXE001 catches exactly this, and CI runs on
Linux where the rule applies -- it is skipped on Windows and WSL, so a
developer working there cannot see it locally.

what:
- Record the file as mode 755, matching the other runnable scripts

Author: tony