Fix constant() enum case identity for class consts (#5682)

[PurHur]

Summary

• constant('C::X') when C::X = E::A now returns the canonical enum case singleton via EnumCaseSupport::materializeConstantValue() on class-const lookup (VmConstants, VM::copyClassConstOrStaticPropertyByName).
• === between enum cases compares declaring enum FQCN + case name instead of ClassEntry object identity, fixing mismatches between compile-time stub entries and runtime enum classes (declare(strict_types=1) repro).

php-src reference: ext/standard/basic_functions.c (zif_constant), Zend/zend_enum.c (case singleton identity)

PHP implementation: ext/standard/VmConstants.php, lib/VM.php, lib/VM/EnumCaseSupport.php — no runtime/ C changes.

Closes #5682

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php bin/vm.php test/repro-maintainer/parity_constant_class_enum_const.php
./vendor/bin/phpunit --filter constant_class_enum
php bin/compile.php -l test/repro-maintainer/parity_constant_class_enum_const.php
'

Output:

enum(E::A)
same
E
OK (1 test, 1 assertion)
exit=0

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