getlines_common

Common implementation for getlines/getline/gets. ix=0 → getlines (list context, croak if scalar) ix=1 → getline/gets (scalar context)

IO.xs:520-565 constructs a fake UNOP with op_ppaddr=PL_ppaddr[OP_READLINE] and calls CALLRUNOPS. This is deeply tied to perl5’s internal op machinery.

DEVIATION: We use eval_pv to read via <$fh>. The IO.xs XS versions exist to work around a lexical hints propagation issue with perl5’s pure-Perl getline/getlines in IO::Handle.pm. Since our C runtime handles hints natively, this workaround achieves the same result.