Stdlib: mb_ucfirst()/mb_lcfirst() on forward 8.4 profile (#17609)

[PurHur]

Summary

• Register mb_ucfirst() / mb_lcfirst() (PHP 8.3+, php-src ext/mbstring/mbstring.c) on forward PHP_COMPILER_PROFILE=8.4 with function_exists() parity via CompilerVersion::supportsMbUcfirstLcfirst() / advertisesMbUcfirstLcfirst().
• Implement VmMbstring::ulcfirst() (first multibyte char title/lower case) and wire builtins in ext/mbstring/Module.php + BuiltinIntrospectionPolicy.
• Add compliance/repro guards and capability-matrix gated rows; extend self-host spine includes.

Closes #17609

php-src reference

• ext/mbstring/mbstring.c — PHP_FUNCTION(mb_ucfirst), PHP_FUNCTION(mb_lcfirst), php_mb_ulcfirst()

PHP implementation

• ext/mbstring/mb_ucfirst.php, ext/mbstring/mb_lcfirst.php
• ext/mbstring/VmMbstring.php — ucfirst() / lcfirst() / ulcfirst()
• lib/CompilerVersion.php — forward-profile gates

Verification

./script/docker-exec.sh -- bash -lc 'PHP_COMPILER_PROFILE=8.4 php bin/vm.php test/repro/maintainer_gap_mb_ucfirst_forward84_missing.php'
# ok

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter mb_ucfirst'
# OK

php script/bootstrap-inventory.php --check
# OK 4315/4315

php script/capability-matrix.php --check
# docs/capabilities.md is up to date (925 builtins)

php script/check-selfhost-spine-coverage-sync.php
# OK

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