mbstring: mb_detect_encoding() VM parity (#3075)

[PurHur]

Summary

• Implement mb_detect_encoding() in ext/mbstring/ via VmMbstring::detectEncoding() — php-src ext/mbstring/mbstring.c UTF-8 preference when valid multibyte UTF-8 competes with ISO-8859-1 in the candidate list.
• Register builtin in ext/mbstring/Module.php; completes the remaining slice of Stdlib: mb_convert_encoding / mb_detect_encoding (ext/mbstring parity) #3075 alongside existing mb_convert_encoding().
• Add compliance .phpt, maintainer repro, and spine require for ext/mbstring/mb_detect_encoding.php (inventory 4403/4404).

php-src reference

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

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php bin/vm.php test/repro/maintainer_gap_mb_detect_encoding.php
vendor/bin/phpunit --filter mb_detect_encoding
php script/check-selfhost-spine-coverage-sync.php
'

ISO-8859-1
ISO-8859-1
ok
OK (2 tests, 2 assertions)
check-selfhost-spine-coverage-sync: OK (spine covers all 4404 inventory files)

Closes #3075

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