Tolerate composite primary keys

[violet-graves]

partially closes #291

we still need to figure out how we want inspection to work

[jas14]

We discussed this in Slack, but just so we remember where we left off, a couple things:

Re: test failures, although Rails 7.0 went EOL in April 2025, if we can easily run the composite primary key tests only in Rails 7.1+, that would be a bit nicer for people than bumping the minimum to 7.1.

Re: inspection, I think it would look nicer to display the primary key fields sorted first and flattened, like so:

#<SuperDiff::Test::Models::ActiveRecord::Order store_id: 456, sku: "hi", details: nil, ...>

versus this, in which the keys and values aren't visually colocated:

#<SuperDiff::Test::Models::ActiveRecord::Order ["store_id", "sku"]: [456, "hi"], details: nil, ...>

Lastly, we're still emitting the primary key fields twice; we should only do that once.