Improve pure Ruby selector compatibility on JRuby

[samuel-williams-shopify]

Summary

• improve the pure Ruby selector path for JRuby by using nonblocking read/write helpers and explicit nonblock state restoration
• add targeted JRuby skips for VM/platform gaps around nested fiber transfer, closed-IO scheduler interactions, FIFO channels, TCP OOB priority readiness, and UNIXSocket buffer sizing

Verification

• bundle exec rubocop
• bundle exec bake test
• GitHub Actions JRuby job is passing on branch interrupt-fix (98b204b)

Note: Windows jobs are still failing separately and were not part of this JRuby-focused pass.

[samuel-williams-shopify]

Superseded by #193, which recreates the JRuby compatibility work on a fresh branch from main without the old interrupt-fix diagnostic history.