Improve JRuby selector compatibility

[samuel-williams-shopify]

Summary

• improve pure Ruby selector compatibility with JRuby by avoiding unsupported IO::Buffer operations and using nonblocking Ruby IO methods where needed
• make Selector.nonblock restore nonblocking state explicitly and ensure the helper is loaded for the pure Ruby selector
• add targeted JRuby skips for VM/platform gaps around nested fiber transfer, closed-IO scheduling, FIFO channels, TCP OOB priority readiness, and UNIXSocket buffer sizing
• keep the branch based directly on main, without the old fork diagnostic workflow/test commits

Verification

• bundle exec rubocop
• bundle exec bake test

Note: this replaces the old interrupt-fix PR branch.

[samuel-williams-shopify]

cc @headius

[headius]

I'll jump in to help where I can!

[ioquatix]

@headius I filed a few upstream bug reports on JRuby.

IO::Buffer is not set in stone yet, but IO::Buffer#read and #write may not follow the current spec which is making things more tricky than it should be.

Also, what's the current state of Loom/io_uring etc? Just wondering if it would make sense to create a JNI for better IO. How crazy would that be?