Fix ??= on property hooks — backing null-check + get/set hooks (#6472)

[PurHur]

Summary

• ??= on hooked properties now null-checks backing storage (via propertyHookRegistry set/get backing fields), not the get-hook return value — matching Zend when backing is null but get returns a default.
• Coalesce lowering emits property fetch read on the left branch and property fetch write + assign on the right branch so VM routes through get/set hooks (dispatchPropertySetHookAssign).
• Adds compliance PHPT covering assign-when-null and no-op-when-backing-set.

php-src reference

• Zend/zend_property_hooks.c — coalesce assign on hooked properties

PHP implementation

• lib/Compiler.php — compileCoalesce property fetch read/write branches + issetForCoalesceAssign flag
• lib/VM.php — objectPropertyIsSetForCoalesceAssign()
• lib/OpCode.php — issetForCoalesceAssign on TYPE_ISSET

Closes #6472

Verification

Issue repro:

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
cat > /tmp/hook_nullcoalesce.php <<'"'"'PHPEOF'"'"'
<?php
class C {
    private ?string $x = null;
    public string $y { get => $this->x ?? "default"; set => $this->x = $value; }
}
$c = new C();
$c->y ??= "assigned";
echo $c->y, "\n";
PHPEOF
php bin/vm.php /tmp/hook_nullcoalesce.php'

Output: assigned

Compliance:

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter property_hook_nullcoalesce_assign test/compliance/VMTest.php'

Output: OK (1 test, 1 assertion)

Regression (related coalesce + property hooks):

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter "property_hook_nullcoalesce_assign|property_hook_set_arrow|coalesce_assign|property_hook_unset" test/compliance/VMTest.php'

Output: OK (9 tests, 9 assertions)

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