Fix enum case === / == regression when constants store backing scalars (#5832)

[PurHur]

Summary

• Materialize enum case objects on class const fetch using enumCases metadata when enumCaseCanonicalNames or stored values are legacy backing scalars (Language: backed enum case === / == with backing scalar must be false (zend_operators.c) #5798 regression).
• Add compile-time isEnumCaseDeclare fallback (enum case Const_ uses flags=0; user const carries visibility bits) so folds emit enum case objects when php-cfg isEnumCase is missing.
• Add maintainer repro script and unit regression test for legacy scalar constant tables.

php-src reference

• Zend/zend_operators.c — enum case vs backing scalar compare is never equal
• Zend/zend_enum.c — enum case singleton materialization

Verification

./script/docker-exec.sh -- bash -lc 'php bin/vm.php test/repro/maintainer_enum_compare_regression_5798.php'
# int-ident:false ... str-equal:false

./script/docker-exec.sh -- bash -lc 'php vendor/bin/phpunit --filter "VmEnumCompareBackingScalarRegressionTest|enum_compare_backing_scalar"'
# OK (6 tests, 12 assertions)

Legacy scalar repro (was true, now false):

./script/docker-exec.sh -- bash -lc 'php -r "
require \"vendor/autoload.php\";
\$runtime = new PHPCompiler\\Runtime();
\$runtime->run(\$runtime->parseAndCompile(\"<?php enum E: int { case A = 1; }\", \"d.php\"));
\$e = \$runtime->vmContext->classes[\"e\"];
\$s = new PHPCompiler\\VM\\Variable(PHPCompiler\\VM\\Variable::TYPE_INTEGER); \$s->int(1);
\$e->constants[\"a\"] = \$s; unset(\$e->enumCaseCanonicalNames[\"a\"]);
ob_start(); \$runtime->run(\$runtime->parseAndCompile(\"<?php var_export(E::A === 1);\", \"c.php\")); echo ob_get_clean();
"'
# false

Closes #5832

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