Add resolved Justfile generator

[darkn3rd]

Summary

• add a resolved Justfile backend that reuses the existing manifest resolver and install command renderer
• validate Ubuntu, macOS, native Windows, MSYS2, and Cygwin host environments, with an explicit cross-generation override
• add root Just recipes for generating and running per-platform installation plans
• document generated Justfile usage and add focused coverage for provider selection and Windows elevation

Testing

• ruby scriptbox/scripts/test/test_generate_justfile.rb
• ruby scriptbox/scripts/test/test_generate_install_script.rb
• ruby scriptbox/scripts/test/test_resolve_order.rb
• 80 tests, 197 assertions, 0 failures