Language: clone() on enum cases throws Error (#3554)

[PurHur]

Summary

• E::Case enum fetches now materialize a VM EnumCaseEntry (minimal Zend case object) instead of bare scalar constants, preserving backed-enum echo via Variable::toString().
• TYPE_CLONE detects enum case operands and dispatches a catchable Error with message Cannot clone enum case Class::Case via BuiltinExceptionSupport::materializeError().
• Registers Throwable as parent of builtin Error so catch (Throwable $e) matches (distinct from readonly-object clone rules in Language: clone() on readonly objects — deep copy + readonly property guards (Zend zend_clones.c parity) #3430).

php-src reference

• Zend/zend_clones.c — enum clone_obj guard / uncloneable object error
• Zend/zend_enum.c — case object creation on E::Case fetch

Verification

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

Closes #3554

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