fix: preserve string portion of $! for unknown error messages

[fglock]

Summary

Fixes a $! dualvar bug discovered while running ./jcpan -t File::Digest.

ErrnoVariable.set(String) was discarding the string and storing type=INTEGER, value=0 whenever the input was neither a number nor a known strerror() message. This is the path SystemOperator takes when it sets $! from a Java IOException message (e.g. after system of a nonexistent program). As a result, my $str = $! saw 0 instead of the message, breaking dualvar semantics.

Reproducer (before fix)

$ ./jperl -e 'system "/tmp/nonexistent"; my $str = $!; print "[$str]\n"; print $str ? "TRUE\n" : "FALSE\n"'
[0]
FALSE

System Perl correctly returns [No such file or directory] / TRUE.

Fix

Store the unknown-string branch as DUALVAR (numeric side RuntimeScalar(0), string side the message), matching the other set(...) paths in the same file. Numeric ops still take the fast path through DualVar.numericValue(), so they don't go through NumberParser.parseNumber() and don't emit "isn't numeric" warnings — preserving the intent of 7124842.

Impact

Unblocks PERLANCAR/Proc-ChildError t/01-basics.t, which does:

system "/tmp/nonexistent...";
like(explain_child_error(), qr/^failed to execute: \S.+ \(-1\)/);

This in turn unblocks much of the File::Digest dependency chain under ./jcpan -t File::Digest:

• Proc::ChildError — PASS (was FAIL)
• IPC::System::Options — PASS
• Perinci::Object — PASS
• File::Digest — PASS

Remaining failures in the chain (JavaScript::QuickJS, parts of Data::Sah::Coerce, Time::Moment) are unrelated XS/native-binding issues.

Test plan

• make passes (all unit tests green)
• ./jperl -e 'system "/tmp/nonexistent"; my $str = $!; ...' now matches system Perl's behaviour
• ./jcpan -t Proc::ChildError — Result: PASS
• ./jcpan -t File::Digest — File::Digest itself: Result: PASS

Generated with Devin