Preserve undef lvalue aliases through goto wrappers #1915

Summary

Sub::Quote 2.006009 fails on PerlOnJava when Sub::Defer creates an attributed :lvalue wrapper. The wrapper uses goto &$undeferred to forward a call to a deferred subroutine. When the deferred sub returns a writable lexical slot whose current value is undef, PerlOnJava rejects the return as though it were an invalid empty/undef lvalue return.

The affected CPAN run is 20261028-144340-47138, failure row Sub::Defer from Sub::Quote. The three affected test programs are t/sub-defer-no-subname.t, t/sub-defer.t, and t/sub-quote.t; each reaches all its assertions before process exit 255 with Can't return undef from lvalue subroutine at Sub/Defer.pm line 163.

Reproducer

my $slot;
sub target :lvalue { $slot }
sub wrapper :lvalue { goto &target }
wrapper() = 'assigned';

System Perl accepts the assignment and updates $slot. PerlOnJava rejects it on both JVM and interpreter backends. The behavior is distinct from a genuinely invalid empty scalar lvalue return, which must continue to report Can't return undef from lvalue subroutine.

Evidence

• Sub-Quote 2.006009 contains pure Perl; no XS/native dependency is involved.
• System Perl 5.34.1: complete focused upstream suite passes (15 files, 6,546 tests).
• PerlOnJava parent c7c805b7aca9bb1ce5effb5f151ccea5caaa48a8: the focused reproducer fails on both backends.
• Environment: macOS arm64, OpenJDK 24.0.2.
• Prior lvalue-subroutine compatibility work in fix: complete lvalue subroutine return compatibility #1479 added validation for genuine invalid undef returns; this report concerns forwarding a writable undef slot through goto.
• The related thread-identity issue Sub::Quote deferred subs lose identity across threads #1420 was fixed separately by fix: preserve fieldhash and deferred sub identity in threads #1761 and is not part of this behavior.

Expected behavior

A :lvalue subroutine using goto &target must preserve the target's writable scalar cell, including when that cell currently contains undef, on both execution backends. Invalid empty and readonly lvalue returns must remain rejected.