WIP: ob read API AOT prep — array_pop fix + lazy stack link (#4914)

[PurHur]

Summary

Partial progress toward #4914 (JIT/AOT lowering for ob_get_contents / ob_get_length / ob_end_clean).

• PHP-in-PHP: ObOutputJitHelper no longer uses array_pop() — manual popStackTop() avoids nested-JIT misroute to HashTable::popLast() (Stdlib: system()/passthru() bypass active output buffer — ob_get_clean() empty (ext/standard/exec.c) #10492), unblocking full ObOutputJitBridge stack compilation later.
• JIT lowering: JitObGetContents / JitObGetLength / JitObEndClean / JitObGetLevel now call ObOutputRuntime::ensureObStackLinked() before ABI emission (parity with JitObGetClean).

Blocker (AOT user-script)

JIT is already green. AOT bin/compile.php (sets PHP_COMPILER_AOT_USER_SCRIPT=1) still fails:

• Link: read/discard __phpc_ob_* symbols undefined — ObOutputUserScriptLlvm only emits echo bridges.
• Full stack nested JIT: ObOutputJitBridge::implementObStack module-verify fails on STDOUT stream filter during inline helper compile.

Investigation notes on the issue: #4914 (comment)

Verification

# JIT (green)
HARNESS_DOCKER_RUN_OPTS='--memory=8g --cpus=2' ./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

# AOT (still red — link)
HARNESS_DOCKER_RUN_OPTS='--memory=8g --cpus=2' ./script/docker-exec.sh -- bash -lc \
  'source script/php-env.sh && vendor/bin/phpunit --filter ob_get_contents'

Refs #4914 — not ready to merge until AOT fixture test/fixtures/aot/cases/ob_get_contents.phpt passes.

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