This change was modified a bit after cherry-picking, to no longer
include the broken changes to item modeling in 9eaee4a.
(cherry picked from commit 90407403ba)
Because we ended up with such a big error, and it doesn't have an easy
fix, I'm wrapping up today by reverting the entire set of refactors
we've done lately, so modeling in production can continue while we
improve this code further over time.
I generated this commit by hand-picking the refactor-y commits
recently, running `git revert --no-commit <hash>` in reverse order,
then manually updating `pet_spec.rb` to reflect the state of the code:
passing the most important behavioral tests, but no longer passing one
of the kinds of annoyances I *did* fix in the new code.
```shell
git revert --no-commit 48c1a58df9
git revert --no-commit 42e7eabdd8
git revert --no-commit d82c7f817a
git revert --no-commit 5264947608
git revert --no-commit 90407403ba
git revert --no-commit 242b85470d
git revert --no-commit 9eaee4a2d4
git revert --no-commit 52ca41dbff
git revert --no-commit c03e7446e3
git revert --no-commit f81415d327
git revert --no-commit 13ceec8fcc
```
This bug never made it into production I think, it was a consequence of
some of how I refactored stuff in the recent changes? I think??
But yeah, I refactor how we manage `SwfAsset#body_id`, to be a bit more
explicit about when and how it can change, instead of the weird
callbacks that tbqh have bit us too often…
