php-in-php: route JIT ob_* stack through ObOutputJitHelper not LLVM monolith (#9268)

[PurHur]

Summary

• Add ext/standard/ObOutputJitHelper.php as PHP SSOT for JIT/embed output-buffer stack (mirrors lib/VM/OutputBuffer.php semantics).
• Replace the ~1,087-line ObOutputRuntime.php LLVM monolith with a 28-line delegator + ObOutputJitBridge.php thin trampolines (NestedJitCompileScope → compiled PHP helpers).
• Quarantine standalone AOT LLVM in ObOutputStandaloneLlvm.php (same pattern as ZlibRuntime / StringZlibJit).

LLVM shrink (JIT/embed path): ObOutputRuntime.php 1087 → 28 lines; stack logic moved to PHP (ObOutputJitHelper 263 lines) + bridge (620 lines). No ObStorageGlobals in JIT bridge.

php-src ref: ext/standard/output.c

Verification

# Issue repro — VM + JIT match Zend
./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php bin/vm.php -r "
ob_start(); echo \"hello\"; \$buf = ob_get_contents(); ob_end_clean();
var_dump(\$buf); echo ob_get_level(), \"\\n\";
"
php bin/jit.php -r "
ob_start(); echo \"hello\"; \$buf = ob_get_contents(); ob_end_clean();
var_dump(\$buf); echo ob_get_level(), \"\\n\";
"'
# string(5) "hello" / 0 on both

vendor/bin/phpunit test/unit/ObOutputRuntimeShrinkTest.php
# OK (3 tests, 20 assertions)

vendor/bin/phpunit test/unit/JIT/ObOutputRuntimeStandaloneTest.php
# OK (1 test, 17 assertions)

php script/check-selfhost-spine-coverage-sync.php
# OK

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

Closes #9268

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