Stdlib: mb_encode_mimeheader()/mb_decode_mimeheader() RFC 2047 parity (#6038)

[PurHur]

Summary

• Implement mb_encode_mimeheader() and mb_decode_mimeheader() in ext/mbstring/ with RFC 2047 B/Q encoded-word support (php-src ext/mbstring/mbstring.c).
• PHP-in-PHP: VmMbstring::encodeMimeheader() / decodeMimeheader(); compile-time JIT/AOT fold via JitMbMimeheader.php; enum operands throw TypeError via shared string guards.
• Compliance + AOT fixtures guard round-trip, mixed ASCII/non-ASCII, Q encoding, and php-src-strict enum rejection.

Closes #6038

php-src reference

• ext/mbstring/mbstring.c — PHP_FUNCTION(mb_encode_mimeheader), PHP_FUNCTION(mb_decode_mimeheader), mb_mime_header_encode, mb_mime_header_decode

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php bin/vm.php -r "
echo function_exists(\"mb_encode_mimeheader\") ? \"encode yes\n\" : \"encode no\n\";
\$s = \"日本語\";
\$enc = mb_encode_mimeheader(\$s, \"UTF-8\");
echo \$enc, \"\n\";
echo mb_decode_mimeheader(\$enc), \"\n\";
echo mb_encode_mimeheader(\"Hello 世界\", \"UTF-8\"), \"\n\";
"
vendor/bin/phpunit --filter mb_encode_mimeheader
'

Output:

encode yes
=?UTF-8?B?5pel5pys6Kqe?=
日本語
Hello =?UTF-8?B?5LiW55WM?=
OK (6 tests, 10 assertions)

Test plan

• VM repro matches Zend for UTF-8 round-trip, ASCII passthrough, mixed strings
• vendor/bin/phpunit --filter mb_encode_mimeheader — 6/6 (VM + JIT + AOT + enum TypeError)
• Capability matrix rows: mb_encode_mimeheader, mb_decode_mimeheader (yes/yes/yes)

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