Ruby threads have been normal native threads up to now. This patch
allows opting into using virtual threads for Ruby threads, in a
model closer to m:n threading.

New property: jruby.thread.vthread=(true/*false)

Replace the regression test's fixed default assumption and repeated startup
program with JRuby-specific behavior examples in explicitly configured fresh
JVMs. Cover backing identity, lifecycle, priority inheritance and deferred
interrupt delivery independently of the parent suite's thread mode.

Run native process waits on platform workers while preserving errno and
interrupt handling on the Ruby caller. Park fiber creators until their
backing thread is adopted instead of repeatedly yielding.

Cover process status, detach, errno and concurrent fiber results in fresh
JVMs with explicitly enabled and disabled virtual Ruby threads and bounded
carrier pools.

Use the same intrinsic monitor for all weak subclass-set operations.
A parkable lock can unmount a waiter that later needs a carrier while
other threads pin all carriers under the hierarchy monitor.

Preserve mutual exclusion for weak-map cleanup during reads. Existing
class-concurrency coverage passes repeated bounded-carrier controls and
MRI Class/Module tests in both Ruby thread backing modes.

Scheduler close can resume fibers and wait for them. Holding the thread
monitor pins JDK 21 carriers during this handoff and can prevent the
fibers from ever running.

Guard disposal entry separately and publish completion after scheduler
close, keeping scheduler joiners blocked until teardown is complete.
Finish cleanup in a finally block without suppressing callback errors.

Add fresh-JVM coverage for scheduler shutdown with explicit virtual and
platform Ruby threads. The enabled case detects the old deadlock with a
single carrier; the disabled control passes.

Restore the fiberBlock parameter name used by upstream frame cloning.
The merge retained the old block parameter, duplicating the local block
variable and leaving fiberBlock undefined, so all JRuby builds failed.

Keep both per-fiber frame cloning and the CompletableFuture startup
handoff from the virtual-thread fixes.

Release the channel blocking monitor across selector waits and restore
its original blocking mode only after the final concurrent selector is
deregistered. Run blocking Java selector calls on platform workers for
virtual Ruby threads, keeping Ruby event polling and cleanup on the caller.

Cover concurrent accepts, targeted interruption, server reuse and closing
pending acceptors in fresh JVMs with explicit backing modes.

Preserve shared-channel selection and virtual-thread selector offloading
while taking currentSelector away under its wakeup lock before pooling.
Retain upstream monitor wakeups and monotonic thread-join behavior.