Stdlib: imap mailbox list/subscribe/create local-mbox surface (#27799)

[PurHur]

Summary

• Implements imap_list / imap_lsub / imap_subscribe / imap_unsubscribe / imap_createmailbox / imap_deletemailbox / imap_renamemailbox / imap_getmailboxes on the pure-PHP local mbox engine when IMAP is advertised (PHP_COMPILER_ENABLE_IMAP=1 or PROFILE≥8.4).
• php-src refs: ext/imap/php_imap.stub.php, ext/imap/php_imap.c. PHP-in-PHP in ext/imap/ — no new runtime/*.c.
• Closed-handle TypeError matches Zend; directory scan uses c-client * / % patterns.

Closes #27799

Test plan

• Issue repro (VM):

PHP_COMPILER_ENABLE_IMAP=1 php bin/vm.php test/repro/issue_27799_imap_mailbox_mgmt.php

Output: all eight function_exists=yes; create/list/subscribe/lsub succeed; closed connection → TypeError.

• Compliance FILE section via stdin matches --EXPECT-- for test/compliance/cases/stdlib/imap_mailbox_mgmt.phpt
• php script/bootstrap-inventory.php --check → OK 7029/7029
• php script/check-selfhost-spine-coverage-sync.php → OK
• ./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && ./script/check-generated-docs.sh' → all generated docs in sync
• Prior Stdlib: ext/imap — imap_open/mail/message API (ext/imap parity) #3663 repro still green (test/repro/issue_3663_imap_open.php)
• Not covered: JIT/AOT (VM-only v1, same as existing imap_*); full c-client remote LIST

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