Add SplDoublyLinkedList ArrayAccess offset API (#13088)

[PurHur]

Summary

• Registers offsetExists, offsetGet, offsetSet, and offsetUnset on SplDoublyLinkedList.
• Bracket read/write/unset delegates to the deque backing store with OutOfRangeException for invalid indices and ?int coercion matching php-src.

php-src reference: ext/spl/spl_dllist.c — spl_dllist_object_* offset handlers.

Closes #13088

Verification

./script/docker-exec.sh -- bash -lc 'php bin/vm.php test/repro/maintainer_gap_spldoublylinkedlist_arrayaccess.php'
# ok

./script/docker-exec.sh -- bash -lc 'php bin/vm.php test/compliance/cases/stdlib/spl_dllist_arrayaccess.phpt'
# 1 / true / 99 / 1 / 99

Note: foreach over SplDoublyLinkedList still requires Iterator method registration (separate from this ArrayAccess slice).

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