fix: make unnecessary commit work less likely (#18263)

While looking at
https://github.com/sveltejs/svelte/issues/18221#issuecomment-4497918414
and trying to understand how the invariant can happen I noticed that we
are not correctly filtering during commit.
- we were not ignoring deriveds
- we were not comparing the correct values (checking `source.v` instead
of the saved value) and not checking if their "is a derived" state
differs

I'm not able to come up with a test where something fails without these
(possibly because it's more about an optimization to do less reruns and
not about correctness) fixes, but they do make sense.