[PurHur]

Summary

• Route base64_encode() / base64_decode() runtime JIT/AOT through Base64EncodeJitHelper / Base64DecodeJitHelper delegating to VmString (php-src ext/standard/base64.c SSOT in PHP).
• Add lib/JIT/Builtin/StringBase64Encode / StringBase64Decode nested-JIT bridges; retain JitBase64Encode / JitBase64Decode LLVM only for user-script standalone AOT defer (Stdlib/AOT: standalone binary run segfault (exit 139) after compile — Docker harness JIT-linked builtins #16734).
• Hot path no longer emits ~437 lines of inline LLVM per call site; defer path unchanged.

Closes #17234

C runtime shrink

• No new C TUs; LLVM monolith bypassed on hot path.
• ext/standard/JitBase64Encode.php (204 LOC) and JitBase64Decode.php (233 LOC) retained only for defer/standalone LLVM fallback.

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter Base64RuntimeShrinkTest'
# OK (4 tests, 22 assertions)

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter "Base64BuiltinTest|Base64RuntimeShrinkTest"'
# OK (6 tests, 32 assertions)

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php bin/vm.php -r "echo base64_encode(\"foo\"), \"\\n\"; echo base64_decode(\"Zm9v\"), \"\\n\";"'
# Zm9v
# foo

php script/check-selfhost-spine-coverage-sync.php
# check-selfhost-spine-coverage-sync: OK (spine covers all 4228 inventory files)

make bootstrap-selfhost-vm-driver-execute-probe
# bootstrap-selfhost-vm-driver-execute-probe: OK

Note: AotTest enum-case base64_*_enum_case_type_error fixtures still fail on master with HashTable::append() during helper-runtime cache hit — pre-existing, unrelated to this bridge.

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