define() isolation breaks any app using require_once for files that
call define() (WordPress, Laravel, Symfony — virtually all PHP apps).
require_once is a no-op on request 2, so cleaned constants are never
re-defined → "Undefined constant" fatals.

The isolation primitives (zealphp_define_hook, zealphp_constants_clear,
zealphp_globals_snapshot/clean, zealphp_process_state_snapshot/clean)
remain available for explicit use by:
- The CGI pool worker (fresh process state per iteration)
- Apps that manage their own define() lifecycle

Co-authored-by: Sibidharan <sibidharan@icloud.com>