Quick post for future me:

Jekyll was being incredibly painful on macOS this morning.

Installing eventmachine 1.2.7 with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

and

Could not find concurrent-ruby-1.1.7 in any of the sources
Run `bundle install` to install missing gems.

and

listen-3.2.1 requires ruby version >= 2.2.7, ~> 2.2, which is incompatible with
the current version, ruby 3.0.0p0

The fix was surprisingly simple:

bundle update

Which looked good but the bundle exec jekyll serve still crashed with:

cannot load such file -- webrick (LoadError)

so we add:

bundle add webrick

and then the bundle exec jekyll serve is back on track.

One qualification for future me: bundle update was the fix for this particular 2021 dependency tangle. It re-resolves dependencies and can change the lockfile quite a bit. For a fresh checkout that already has a working lockfile, start with bundle install. If an update is needed, check the diff before committing it. Bundler’s update reference covers the difference.