JIT: Enum::cases() lowering (#4068)

[PurHur]

Summary

• Lower synthetic Enum::cases() as a per-enum LLVM function (suit::cases) that returns a packed list of enum case objects built from compile-time case constants.
• Enum case objects use two property slots (name string + backing __value__); ->name / ->value are handled in JIT property fetch.
• php-src reference: Zend/zend_enum.c (zend_enum_list_cases).

Closes #4068

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php vendor/bin/phpunit test/unit/EnumCasesJitCompileTest.php'

Output:

OK (1 test, 2 assertions)

LLVM module contains suit::cases and passes compileCommon() verify.

VM compliance (unchanged):

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php vendor/bin/phpunit test/compliance/EnumVMTest.php --filter enum_cases'
# OK (1 test)

Blocker (not merged)

MCJIT execute still segfaults for scripts that declare user enums (#3518 — same on master for enum E: string {} with no body). JITTest enum_cases remains skipped until execute is stable. Compile-time lowering is complete and guarded by EnumCasesJitCompileTest.

Issue repro via bin/jit.php hits this execute crash after successful compile (previously failed at compile with undefined cases()).

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