Stdlib: debug_zval_dump() reference refcount on aliased array slots (#4709)

[PurHur]

Summary

• Extract VmDebugZval SSOT from debug_zval_dump.php for Zend zend_debug_zval() parity (Stdlib: debug_zval_dump() — zval introspection for debugging (ext/standard/php_debug.c parity) #4709).
• When array elements are aliased via by-ref ($b = &$a[0]), render reference refcount(N) { … } blocks matching php-src output.
• Count external TYPE_INDIRECT wrappers sharing the storage cell; refcount = aliases + 1 (slot).

Root cause

VM stores by-ref values in the array bucket directly; only external variables use TYPE_INDIRECT. debug_zval_dump() previously resolved values and never detected shared storage.

php-src reference

• ext/standard/php_debug.c — PHP_FUNCTION(debug_zval_dump)
• Zend/zend_variables.c — zend_debug_zval() refcount formatting

PHP implementation

• ext/standard/VmDebugZval.php — dump + alias counting via VM::visitStrongRefRoots()
• ext/standard/debug_zval_dump.php — thin builtin delegating to VmDebugZval

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php test/repro-maintainer/debug_zval_dump_refcount.php
php bin/vm.php test/repro-maintainer/debug_zval_dump_refcount.php
# diff empty — VM matches Zend
php test/repro-maintainer/debug_zval_dump_refcount.php | diff -u - <(php bin/vm.php test/repro-maintainer/debug_zval_dump_refcount.php)
vendor/bin/phpunit --filter debug_zval_dump_refcount test/compliance/
'

Output:

array(2) refcount(2){
  [0]=>
  reference refcount(2) {
    int(1)
  }
  [1]=>
  int(2)
}
int(1)
MATCH
OK (3 tests, 3 assertions)

Closes #4709

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