Add mb_str_split() UTF-8/ASCII chunking (#3299)

[PurHur]

Summary

• Implement mb_str_split() in ext/mbstring/ via VmMbstring::strSplit() — UTF-8 character chunks and ASCII/8BIT byte chunks per php-src ext/mbstring/mbstring.c
• Register builtin in mbstring module; ValueError when $length <= 0
• Add repro + compliance guard; refresh capability matrix

Closes #3299

php-src reference

• ext/mbstring/mbstring.c — PHP_FUNCTION(mb_str_split)

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php test/repro/maintainer_gap_mb_str_split.php
php bin/vm.php test/repro/maintainer_gap_mb_str_split.php
'

Output (Zend and VM identical):

array ( 0 => 'α', 1 => 'β', 2 => 'γ', )
array ( 0 => 'ab', )
array ( 0 => 'he', 1 => 'll', 2 => 'o', )
array ( )
length_zero=ValueError

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

./script/ci-fast.sh --filter mb_str_split
# green (capability matrix regenerated)

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