AOT: fix mb_trim NestedJIT rtrim wrong output (leftover #34396)

[PurHur]

Summary

• Fix MbTrimJitHelper AOT wrong output for runtime haystack mb_trim/mb_ltrim/mb_rtrim (leftover of AOT: mb_trim NestedJIT for runtime strings (#34379) #34388/AOT: fix mb_trim NestedJIT ABI (SIGSEGV/empty) leftover of #34388 #34396).
• Replace \substr($s,$i,1) with byte indexing ($value[$i]) and copyPrefix() loop — NestedJIT substr mis-fires under thin AOT.
• Rewrite rtrim NBSP handling with C2-hold deferral instead of $last = $i - 1 / $last === $n - 1 — those icmp/sub patterns miscompile the whole loop under AOT.

Repro (before)

./script/docker-exec.sh -- bash -lc 'export PHP_COMPILER_PROFILE=8.4 PHP_COMPILER_HELPER_RUNTIME_O=0; php bin/compile.php -o /tmp/t.bin test/repro/aot_mb_trim_runtime.php && /tmp/t.bin'
# AOT: h / hi\t / (space) / x … vs VM/Zend: hi / hi\t / ␠␠hi / x …

Verification

./script/aot-smoke.sh
# aot-smoke: 8 passed, 0 failed

./script/phpunit.sh --filter MbTrimRuntimeAotTest
# OK (2 tests, 14 assertions)

./script/docker-exec.sh -- bash -lc 'export PHP_COMPILER_PROFILE=8.4 PHP_COMPILER_HELPER_RUNTIME_O=0; php bin/vm.php test/repro/aot_mb_trim_runtime.php; php bin/compile.php -o /tmp/t.bin test/repro/aot_mb_trim_runtime.php && /tmp/t.bin'
# VM and AOT output identical (6 lines each)

php-src reference

• ext/mbstring/mbstring.c — PHP_FUNCTION(mb_trim) / default charset table

Closes #34379

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