Cool. What would be the steps from a fresh Debian to get it running and see it in action?

Not sure debian has (a modern) ruby installed by default, but I think you only need to have a working ruby installation and running the script. On a sidenote, this came out recently and could interest you https://dashbit.co/blog/announcing-phoenix-playground

I tried it like this:

    1: apt install ruby
    2: Put your code in test.rb
    3: ruby test.rb

I then I got:

    Gem::Ext::BuildError: ERROR: Failed to build gem native extension.
    \e[0m
        current directory: /var/lib/gems/3.1.0/gems/stringio-3.1.1/ext/stringio
    /usr/bin/ruby3.1 -I /usr/lib/ruby/vendor_ruby -r ./siteconf20240626-4044-iylnuy.rb extconf.rb
    mkmf.rb can't find header files for ruby at /usr/lib/ruby/include/ruby.h

    You might have to install separate package for the ruby development
    environment, ruby-dev or ruby-devel for example.

So I tried:

    4: apt install ruby-dev
    5: ruby test.rb

Then I got:

    [4147, #<Thread:0x00007f9a4af6bc60 run>, #<NameError: uninitialized constant Gem::Source
    (defined?(@source) && @source) || Gem::Source::Installed.new
                                         ^^^^^^^^