Fix duplicate backed enum values at compile time (#9677)

[PurHur]

Summary

• Extend EnumBackedCaseCheck::validate() to reject backed enum declarations whose case initializers are compile-time duplicate int/string scalars.
• Align compliance guards and unit tests with compile-time fatal (exit 255) instead of lazy first-access Error for literal duplicates.

php-src reference

• Zend/zend_enum.c — zend_register_enum_case duplicate backing detection

PHP implementation

• lib/Compiler/EnumBackedCaseCheck.php — compile-time duplicate backing scan

Verification

php bin/vm.php test/repro/enum_duplicate_value_echo.php
# Fatal error: Duplicate value in enum E for cases A and B (exit 255)

vendor/bin/phpunit test/unit/EnumBackedCaseCheckTest.php
# OK (9 tests, 14 assertions)

vendor/bin/phpunit test/compliance/EnumVMTest.php --filter 'enum_duplicate_backing_value|duplicate_enum_backing_value|enum_duplicate_backing_value_compile'
# OK (3 tests, 3 assertions)

vendor/bin/phpunit test/compliance/DuplicateEnumBackingJITTest.php
# OK (1 test, 1 assertion)

Closes #9677

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