Stdlib/AOT: ob_get_contents/get_length/end_clean user-script link (#4914)

[PurHur]

Summary

• User-script AOT (PHP_COMPILER_AOT_USER_SCRIPT=1) lowered ob_get_contents(), ob_get_length(), ob_end_clean(), and ob_get_level() but link failed with undefined __phpc_ob_* symbols — JIT/VM were already green via ObOutputJitHelper / nested JIT.
• Extend ObOutputExecCaptureLlvm (php-src ext/standard/output.c parity) with LLVM bridges on the existing ObStorageGlobals char-buffer stack for read/discard ABI symbols.
• Lazy-link from JitObGetContents / JitObGetLength / JitObEndClean / JitObGetLevel through ObOutputRuntime::ensureObStackLinked().

php-src reference

• ext/standard/output.c — php_ob_get_contents, php_ob_get_length, php_ob_end_clean, php_ob_get_level

PHP implementation

• lib/JIT/Builtin/ObOutputExecCaptureLlvm.php — LLVM emit (no new C runtime)
• lib/JIT/Builtin/ObOutputRuntime.php — lazy read-API link for deferred nested-JIT user-script AOT
• ext/standard/JitOb*.php — ensure stack linked before ABI lookup

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php -d memory_limit=1536M vendor/bin/phpunit --filter ob_get_contents'
# OK (3 tests, 7 assertions) — includes AOT fixture ob_get_contents.phpt

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php bin/jit.php -r "ob_start(); echo \"hello\"; \$c=ob_get_contents(); \$l=ob_get_length(); ob_end_clean(); echo \$c,\$l,ob_get_level(),\"\n\";"'
# hello50

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php -d memory_limit=1536M vendor/bin/phpunit --filter ObOutputRuntimeShrinkTest'
# OK (3 tests, 33 assertions)

Closes #4914

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