Runtime: fix user-script AOT ob stack + system()/passthru() capture (#10492)

[PurHur]

Summary

• Ob echo routing: ObOutputUserScriptLlvm now links the exec-capture ob stack and routes echo through ObOutputEchoJitEmit / __phpc_ob_append_bytes instead of EmbedObEchoBridge direct write(1,…).
• Ob stack (defer gate): ObOutputExecCaptureLlvm implements ob_start / append_bytes / ob_get_clean via ObStorageGlobals when UserScriptAotDeferNestedJit is active — nested ObOutputExecCaptureJitHelper JIT segfaults in standalone AOT.
• Exec capture (defer gate): ProcessShellExecLibc provides libc popen/fread __compiler_shell_exec; ProcessExecCaptureLlvm builds the capture hashtable with __string__separate (no buffer alias).

Fixes AOT segfault in system_passthru_ob and restores php-src behavior for child stdout through active output buffers.

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php bin/vm.php test/repro/maintainer_gap_system_passthru_ob.php'
# system_ob_ok=1 / passthru_ob_ok=1

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit test/aot/AotTest.php --filter system_passthru'
# OK (1 test, 5 assertions)

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit test/unit/ObOutputRuntimeShrinkTest.php test/unit/ProcessRuntimeShrinkTest.php'
# OK (7 tests)

Closes #10492

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