php-in-php: route IteratorHelper foreach LLVM through VmIteratorForeach PHP (#10080)

[PurHur]

Summary

• Shrinks lib/JIT/IteratorHelper.php from ~725 lines of inline LLVM to a 54-line trampoline delegating to lib/VM/VmIteratorForeach.php (744 lines, PHP SSOT for hashtable/SplObjectStorage/WeakMap foreach lowering).
• Adds spine require for VmIteratorForeach.php and shrink guard IteratorHelperRuntimeShrinkTest.
• Rebased onto current master; regenerated docs/bootstrap-inventory.md.

C runtime: no new C — net ~671 LLVM lines removed from IteratorHelper (moved to compiled PHP helper path via existing JitVmHelperLink bridge inside VmIteratorForeach).

Closes #10080

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter IteratorHelperRuntimeShrinkTest'
# OK (2 tests, 10 assertions)

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php bin/vm.php test/repro/foreach_jit_vm_agree.php
php bin/jit.php test/repro/foreach_jit_vm_agree.php
'
# k=v / sum=3 on VM and JIT

php script/check-selfhost-spine-coverage-sync.php
# OK (spine covers all 4482 inventory files)

php script/bootstrap-inventory.php --check
# OK 4482/4482

make north-star5-verify-fast
# north-star5-verify: OK (fast)

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