fix(parser): join multi-element subscripts with $; in chained hash deref

[fglock]

Summary

Fixes a bug where chained hash dereference with multi-element subscripts kept only the last element instead of joining with $; (SUBSEP).

$h{-word => 'ou'}       # FETCH("-word\x1cou")  — already correct
$h{a}{-word => 'ou'}    # FETCH("ou")           — BUG, now fixed

Under the hood, $h{a}{b} goes through handleArrowHashDeref (implicit arrow deref). That path evaluated the subscript list in scalar context, which drops all but the last element. The non-deref top-level path already joined multi-element subscripts with $;, so the two disagreed.

Fix: in handleArrowHashDeref, when the HashLiteralNode has more than one element, emit it as a list and join($;, ...), matching the top-level case.

Discovery

Found while investigating jcpan -t Regexp::Common. Its tied hash chains FETCH calls, and patterns like $RE{list}{conj}{-word => 'ou'} rely on -word reaching FETCH.

Test plan

• make (full unit test suite) passes
• Repro script shows FETCH now receives the full joined key at every level
• Regexp::Common test suite improves from 22/73 failed test files to 11/73; completed subtests jump from 116,146 to 140,752 as previously-aborted files (t/test_list.t, t/URI/http.t, t/number/decimal.t, t/zip/us.t, etc.) now run to completion
• t/test_list.t in Regexp::Common: 50/50 pass (was aborting at test 33)

Repro

package T;
sub TIEHASH { my ($c, @d) = @_; bless \@d, $c }
sub FETCH { my ($s,$k)=@_; print "FETCH len=",length($k),"\n"; bless ref($s)->new(@$s,$k), ref($s) }
sub new { my ($c, @d)=@_; my %h; tie %h, $c, @d; \%h }
package main;
my %RE; tie %RE, 'T';
my $x = $RE{list}{conj}{-word => 'ou'};

Before: last FETCH reported len=2. After: len=8, matching real Perl.

Generated with Devin

[fglock]

Superseded by #555 — that PR now includes this commit.