Stdlib: serialize() append-self-ref R: graph matches php-src (#22653)

[PurHur]

Summary

• Align VmSerialize with php-src php_add_var_hash (ext/standard/var.c): scalars/arrays bump the stream index but are not stored for R: lookup; ISREF sets are keyed by the shared target Variable.
• Fixes $a=[1]; $a[]=&$a emitting a:2:{i:0;i:1;i:1;a:2:{i:0;i:1;i:1;R:3;}} (was wrongly R:2 for the inner scalar).
• Keeps empty self-ref (R:2), shared scalar/array refs, and object R:/r: cases matching Zend.
• PHP-in-PHP only (ext/standard/VmSerialize.php); JIT uses the same path via SerializeJitHelper.

Closes #22653

php-src reference

• ext/standard/var.c — php_add_var_hash / php_var_serialize_intern (IS_REFERENCE → goto again)

Verification

php bin/vm.php test/repro/issue_22653_serialize_append_self_ref.php
# a:2:{i:0;i:1;i:1;a:2:{i:0;i:1;i:1;R:3;}}

php bin/jit.php test/repro/issue_22653_serialize_append_self_ref.php
# same

./script/phpunit.sh --filter serialize_append_self_ref
# OK (2 tests, 2 assertions) — VM + JIT compliance

AOT note: exact circular repro needs AssignRef (not implemented in AOT escape lowering). Host AOT serialize([1,1]) also segfaults on current master (c:main_before_php) — pre-existing, not this diff. Shared helper will apply once AssignRef/AOT serialize runtime is healthy.

Gates (pre-claim probe): inventory OK 6454/6454; north-star5-fast red on master (gen-0 manifest mismatch #8713) — unrelated.

Made with Cursor

[PurHur]

Cleanup hygiene

Deleting remote branch fix/22653-serialize-append-self-ref.

Reason: PR merged into master (#22666 / 5e716a8ce); issue #22653 closed; no open PR on this head.

Open PR #22717 (agent/issue-22642-gen0-refresh-oom) and release/v1.1.0-dev left untouched.