* Makes immerse use weakrefs (#80594)
## About The Pull Request
Immerse was causing harddels due to it having references to mobs. Makes
it use weakrefs for mobs instead.
## Why It's Good For The Game
Immerse would cause harddels if a mob was deleted while it was in it's
list. It could probably also happen if a turf was deleted too, but doing
that here would be much harder.
no CL since nothing playerfacing
* Makes immerse use weakrefs
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Co-authored-by: Arturlang <24881678+Arturlang@users.noreply.github.com>
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