build: Windows CI failure on digest gem duplicate loading conflict

[quartzmo]

CI for windows-latest under Ruby 3.4 fails during bundle install:

RuntimeError: Digest::Base cannot be directly inherited in Ruby
     52: C:/hostedtoolcache/windows/Ruby/3.4.9/x64/bin/toys:36:in '<main>'
     ...
      6: C:/hostedtoolcache/windows/Ruby/3.4.9/x64/lib/ruby/gems/3.4.0/gems/bundler-4.0.13/lib/bundler/checksum.rb:17:in 'Bundler::Checksum.from_gem_package'

Explanation

1. For Ruby 3.4.9, Bundler 4.0.13 resolves standard library dependencies from scratch and decides to fetch and compile the standard library gem digest (version 3.2.0) from RubyGems.
2. During the checksum verification phase, Bundler requires digest. Because the gem path is active, it loads the gem's native C extension DLL (digest.so / digest.dll).
3. On Windows, loading duplicate DLLs of a preloaded C extension initializes a separate memory block, resulting in a different address for Digest::Base. The gem's extension fails the subclassing check when instantiating Digest::SHA256.new, throwing: Digest::Base cannot be directly inherited in Ruby.

Proposed Fix

Upgrade the CI check from Ruby 3.4 to Ruby 4.0 per our current maintenance schedule. In Ruby 4.0, standard libraries like digest are fully extracted from the interpreter core, which eliminates the duplicate DLL collision on Windows since only the gem version is loaded.

Note that we must probably also add ostruct as a development dependency.