Stdlib: imap_delete/undelete/expunge + ping/check/status (#27783)

[PurHur]

Summary

• Implements imap_delete / imap_undelete / imap_expunge / imap_ping / imap_check / imap_status on the pure-PHP local mbox engine when IMAP is advertised.
• Registers SA_MESSAGES / SA_RECENT / SA_UNSEEN / SA_UIDNEXT / SA_UIDVALIDITY / SA_ALL for status flags.
• php-src refs: ext/imap/php_imap.stub.php, ext/imap/php_imap.c. PHP-in-PHP — no new runtime/*.c.
• Also adds spine require_once for lib/JIT/ArrayWalkLlvm.php so inventory sync stays green after Regression: AOT array_walk_recursive() segfaults after build — VM/JIT match Zend (ext/standard/array.c) #27632 landed without it.

Closes #27783

Test plan

• PHP_COMPILER_ENABLE_IMAP=1 php bin/vm.php test/repro/issue_27783_imap_delete_status.php
• Compliance imap_delete_status.phpt FILE matches --EXPECT--
• inventory --check OK 7040/7040
• check-selfhost-spine-coverage-sync OK
• Docker check-generated-docs.sh OK
• Not covered: JIT/AOT (VM-only v1); UID sequences; remote STATUS

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