RbConfig::CONFIG["ruby_version"]: full version or API version? #878

rbgo answers RbConfig::CONFIG["ruby_version"] with the full version string (4.0.9 after the bump, 3.4.1 before it), from internal/vm/kernel_introspect.go's rubyVersion.

MRI may answer the API version there instead — 4.0.0 for every 4.0.x — because that key names the library directory (/usr/lib/ruby/<ruby_version>/), which is shared across teeny releases by design. If so, any gem computing a load path from it gets a directory rbgo's own rubylibdir does not match.

Noticed while bumping rubyVersion for #799. Not changed there, deliberately: nothing in ruby/spec asserts this key's value — library/rbconfig/ covers only unicode_version and unicode_emoji_version — so the corpus cannot settle it, and a version bump is not the place to change behaviour on a hunch.

To close this, read MRI's configure.ac/template/rbconfig.rb.tmpl at v4.0.9 for how ruby_version is defined, and check rubylibdir uses the same value. internal/vm/puppet_boot_modified_test.go carries the current expectation and a comment pointing here.

Refs #799