Run specs in parallel, speed up CI, fix concurrency bugs in specs and code

[apiology]

This allows rspec to use more than one CPU. Really useful on modern dev machines, and also useful in CI, where the default runner size has two CPUs.

See https://github.com/apiology/solargraph/pull/27/changes for a more useful diff

To do:

• Track down slow tests and see where time is going
• Track down rspec + rails specs slowness
• Add developer doc on things to expect, how to write specs
• Fix logging caching gems
• Figure out where doc_map vs workspace fix went

Reduces the actual testing time in CI to about a minute using our small workers:

There are also a few fixes here:

• For some reason, the debug level logging I tried to set in rspec wasn't persisting - this seems to do a better job and found a couple of bugs in the process that only reproduce at debug level logging.
• Some tests didn't run individually, but did when run in sequence in the specs. We may see new failures pop up in the future; the trade-off for faster responsiveness is probably going to be less deterministic behavior in the face of buggy tests.

Includes for mergability:

• 2026-01-27 dev branch #1165

Includes this build fix:

• Work around erb bundle problem with RBS pre-release #1168

Includes the fix for this bug, which parallel running seems to trigger (more often?):

• Test verbosity fix #1166

Includes to show combined effect:

• Spec performance fixes #1158

Includes transitively:

• v0.59 branch