TYPE
====
PHP event (extend an existing event's variables)

LOCATION
========
includes/ucp/ucp_pm_compose.php, core.ucp_pm_compose_modify_parse_after
(~line 922, immediately before "if (!count($error) && $submit)").

PURPOSE
=======
core.ucp_pm_compose_modify_parse_after already fires at exactly the
right point for validating a PM before it's sent, and already exposes
$error and $submit. It does not expose $address_list - the recipient
list, already fully built earlier in the same function - so an
extension can reject a PM but cannot inspect who it's actually being
sent to.

USE CASE
========
phpbbmodders/honeypot silently restricts an account to a group that
can't post/reply after it trips a hidden-field honeypot check, and
that group should also only be able to PM staff, not regular members -
but there's no way to inspect the intended recipients from this or any
other PM-composing event, so that half of the restriction can't be
enforced yet. No new event needed, no change to where this fires -
just one more variable in an event that already exists and already
fires in the right place.

VARIABLES
=========

• address_list (array, read-only) - recipients, keyed 'u' (user ids)
    and 'g' (group ids), each mapping id => 'to'|'bcc'

Add to the existing $vars array and docblock:

    * @var  array   address_list        Array of recipients, keyed 'u' (user ids)
    *                                    and 'g' (group ids), each mapping id => 'to'|'bcc'