Mbstring: implement mb_send_mail() VM builtin (#6548)

[PurHur]

Summary

Implements mb_send_mail() in ext/mbstring/ toward php-src parity (#6548).

• MbstringMailProfile — per-mb_language() mail charset/header/body encodings (php-src libmbfl/nls)
• VmMbstring::prepareSendMail() — recipient normalization, RFC 2047 subject encoding, body charset conversion + transfer encoding, MIME header assembly
• mb_send_mail.php — VM builtin registered in ext/mbstring/Module.php; delegates transport to compiler mail() (returns false when sendmail unavailable, matching mail_registered.phpt)
• Compliance guards + unit test for MIME header preparation

php-src reference: ext/mbstring/mbstring.c — PHP_FUNCTION(mb_send_mail)

PHP-in-PHP: no new C runtime; reuses encodeMimeheader(), convertEncoding(), and ext/standard/mail.php.

Verification

source script/php-env.sh
php bin/vm.php test/repro/maintainer_gap_mb_send_mail.php
# ok

vendor/bin/phpunit --filter 'MbSendMailTest|mb_send_mail'
# OK (7 tests, 11 assertions)

php script/bootstrap-inventory.php --check
# OK 4844/4844

Note: ./script/ci-fast.sh spine-sync still reports 14 pre-existing master drift files (sysvsem, convert_uu) unrelated to this diff.

Closes #6548

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