Support/Handle varying essence types

Currently, the plugin only handles logic for filling pouches with one type of essence. There no logic/I haven't tested multiple essence.

You can only have one type of essence per pouch.

Start by reproducing pouch filling with more than one essence type, since the issue says this behavior is untested. Review the plugin's existing essence-filling and pouch-tracking entry points, then verify that a pouch can track varying essence types while preserving its amount and decay behavior.

Written by the indexing model from the issue text.