Language: enum implements interface — case objects + VM dispatch (#3373)

[PurHur]

Summary

• Enum case fetches (Status::Open) now materialize as cached singleton ObjectEntry instances instead of bare backed scalars, enabling instance method dispatch and instanceof against implemented interfaces.
• Built-in enum case properties name and value match Zend semantics; string context (echo/concat) still uses the backed scalar.
• php-cfg parse path for enum … implements … was already wired (Language: Backed enum VM regression — php-cfg Enum_ parse + case value echo (#1356) #3083); this completes VM runtime parity for the issue repro.

php-src reference: Zend/zend_enum.c (enum registration + case objects), Zend/zend_interfaces.c (interface satisfaction), Zend/zend_compile.c (zend_compile_enum implements clause).

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php vendor/bin/phpunit --filter EnumVMTest'
./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php vendor/bin/phpunit --filter VmEnumImplementsInterfaceTest'
./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php vendor/bin/phpunit --filter PhpCfgEnumImplementsParseTest'

Closes #3373

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