Stdlib: mb_strpos() NestedJIT runtime leftover of #27187 (#34146)

[PurHur]

Summary

• Closes Stdlib: mb_strpos() JIT/AOT happy-path leftover of #27187 — runtime args LogicException (ext/mbstring/mbstring.c) #34146 — leftover of Regression: AOT mb_strpos() not lowered — VM/JIT match Zend (ext/mbstring/mbstring.c) #27187: compile-time mb_strpos() fold was green, but runtime (variable) haystack/needle still threw LogicException under thin AOT.
• NestedJIT via MbSearchJitHelper + MbSearchRuntime; int|false boxed like StringStrpos (-1 miss sentinel).
• Helper inlines UTF-8 walk with strlen/ord/substr and range compares — VmString::utf8CharLength / VmMbstring::strpos are silent-0 under NestedJIT thin AOT (same class as runtime mb_strwidth).
• php-src: ext/mbstring/mbstring.c — PHP_FUNCTION(mb_strpos).

Test plan

• Zend / VM / AOT agree on test/repro/issue_34146_mb_strpos_runtime_aot.php (int(1), bool(false), int(6))
• Offset + ASCII encoding AOT match Zend
• ./script/aot-smoke.sh → 8 passed, 0 failed (via ./script/docker-exec.sh)

./script/docker-exec.sh -- bash -lc 'php test/repro/issue_34146_mb_strpos_runtime_aot.php && php bin/vm.php test/repro/issue_34146_mb_strpos_runtime_aot.php && PHP_COMPILER_HELPER_RUNTIME_O=0 php bin/compile.php -o /tmp/mbsp test/repro/issue_34146_mb_strpos_runtime_aot.php && /tmp/mbsp'
./script/docker-exec.sh -- bash -lc './script/aot-smoke.sh'

Not covered: peer *pos/*str family runtime leftovers (same JitMbSearch fold-only gap).

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