fix: preserve IO::Async thread callback results

[fglock]

Summary

• preserve scalar and list callback results returned through IO::Async-style ithread wrappers on JVM and interpreter backends
• add a project-owned regression covering scalar, list, and exception propagation through threads->join
• retire the stale IO::Async notifier-loop three-owner compatibility expectation in favor of native Perl's two-owner result

Validation

• make
• perl src/test/resources/unit/threads_captured_callback_context.t
• timeout 120 ./jperl src/test/resources/unit/threads_captured_callback_context.t
• timeout 120 ./jperl --interpreter src/test/resources/unit/threads_captured_callback_context.t
• make check-links
• timeout 120 ./jperl -e 'print "UAT build ready\\n"'

Follow-up

The project-owned create_thread regression is fixed. The local extracted
IO::Async Routine/Function probes still expose separate 66 result and
fixture-loading failures; they remain under investigation before #1277 can be
closed.