Language: enum case === / == with backing scalar must be false (#5798)

[PurHur]

Summary

• Reject === / == when exactly one operand is an enum case (Variable::identicalTo / equals), matching Zend zend_operators.c — avoids any looseEqual numeric coercion path.
• Add maintainer repro test/repro-maintainer/parity_enum_strict_identity.php and compliance PHPT enum_compare_backing_scalar.phpt.

php-src reference: Zend/zend_operators.c (compare_function enum case vs scalar).

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php test/repro-maintainer/parity_enum_strict_identity.php
php bin/vm.php test/repro-maintainer/parity_enum_strict_identity.php
vendor/bin/phpunit test/compliance/EnumVMTest.php --filter enum_compare_backing_scalar
'

false (x6 lines — int + string backed enums, === and ==, both directions)
PHPUnit OK (1 test)

Closes #5798

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