Stdlib: mb_substr/mb_strpos/mb_strtolower/mb_strtoupper UTF-8 VM (#3239)

[PurHur]

Summary

• Register mb_substr(), mb_strpos(), mb_strtolower(), and mb_strtoupper() in ext/mbstring/ (php-src ext/mbstring/mbstring.c).
• Shared UTF-8 codepoint logic lives in VmMbstring + VmString::utf8CharSubstr() — no new C runtime.
• VM-only v1 (UTF-8 / ASCII / 8BIT); JIT deferred like other mbstring search helpers.

Closes #3239

php-src reference

• ext/mbstring/mbstring.c — php_mb_substr, php_mb_strpos, case conversion wrappers

PHP implementation

• ext/mbstring/mb_substr.php, mb_strpos.php, mb_strtolower.php, mb_strtoupper.php
• ext/mbstring/VmMbstring.php — substr(), strpos(), strtolower(), strtoupper()

Verification

Issue repro:

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php bin/vm.php -r "var_dump(function_exists(\"mb_substr\"));"
php bin/vm.php -r "echo mb_substr(\"café\", 0, 2, \"UTF-8\");"
'

Output:

bool(true)
ca

Repro script + compliance:

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php bin/vm.php test/repro-maintainer/mb_substr_basic.php
./vendor/bin/phpunit --filter mb_substr_utf8 test/compliance/VMTest.php
'

Output:

bool(true)
bool(true)
ca
int(2)
hello
HELLO
OK (1 test, 1 assertion)

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