TYPE
====
PHP event

LOCATION
========
includes/functions_messenger.php, messenger::template(), before
set_template_paths() is called.

(Alternatively, as an extension of
\phpbb\template\twig\twig::set_custom_style()'s existing per-extension
discovery loop, which already walks all_enabled() extensions but only
ever looks under styles/<name>/template|theme/, never
language/<lang>/email/.)

PURPOSE
=======
Let an enabled extension supply its own copy of a core notification email
template (e.g. language/en/email/post_in_queue.txt), so it can customize
the rendered content of an existing core notification without patching
core or duplicating the whole notification type.

USE CASE
========
Originates from a request to add the recipient's email address to the
post_in_queue admin-approval email. Extensions can already add data via
get_email_template_variables(), but nothing resolves email templates from
an extension's own directory, so there's no way to reference that new
data in the email body.

core.notification_message_email technically allows rewriting the
fully-rendered $subject/$msg post-template, but only as opaque string
surgery on already-composed text — no variable placeholder, no
per-language file — not a substitute for template-level customization.

VARIABLES
=========

• template_lang (string, read-only) — language being resolved
• template_dir_prefix (string, read-only) — subdirectory prefix in play
• template_paths (array, MODIFIABLE) — filesystem paths searched for the
    template file; a listener appends its own extension-owned directory
    here