Fix enum cases() static dispatch (#5507)

[PurHur]

Summary

• Always register synthetic Enum::cases() as public static after enum body lowering (EnumSupport::ensureBuiltinCasesMethod), replacing any stale non-static stub.
• Fast-path VM initStaticCallable and JIT static-call checks for cases() on enums (parity with from/tryFrom).
• Add backed-enum spread compliance PHPT and unit smoke.

Closes #5507

php-src reference

Zend/zend_enum.c — zend_enum_list_cases / static cases() on UnitEnum and BackedEnum.

PHP implementation

• lib/VM/EnumSupport.php — builtin cases() registration
• lib/VM.php — static dispatch + post-defineClass finalize
• lib/JIT.php — JIT static-call guard

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit test/compliance/EnumVMTest.php --filter enum_cases'
./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit test/unit/VmEnumBasicTest.php --filter testBackedEnumCasesSpread'

OK (3 tests, 3 assertions)   # EnumVMTest --filter enum_cases
OK (1 test, 1 assertion)   # VmEnumBasicTest::testBackedEnumCasesSpread

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