Investigate Jest's new custom equality matchers

Historically Jest's `toEqual` has caused us grief b/c of recursively diffing entities down to the connection pool, which can end up pegging a CPU for ~several seconds even on a passing assertion.

We've written `toMatchEntity` to avoid this, but it requires teaching programmers when to use it.

Jest just landed custom equality matchers:

https://github.com/facebook/jest/pull/13654

So we can see if that helps. My suspicion is that `toMatchEntity`'s diff output will still be easier to read, so it probably won't completely go away, but if we can de-suck `toEqual`, that would be great too.

Start by reading the linked Jest custom equality matchers proposal and tracing the existing toMatchEntity implementation and its uses. Compare whether the matcher can prevent toEqual from recursively traversing entities while preserving readable diffs; done means a documented recommendation or a concrete follow-up scope.

Written by the indexing model from the issue text.