[fix] Prevent rerender when route state did not change

[dummdidumm]

Fixes #3732 (and maybe some other issue? Could have sworn I saw something similar about props running in another issue)

No test yet, want to confirm first that this is the right fix

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[Rich-Harris]

Ah, interesting. We definitely don't want to re-run load in that case, yeah. That said, there are cases where you do want to trigger a navigation even though nothing changed — think of e.g. you're on / and you click the home icon in the top left, which should cause data to be re-fetched and the scroll to reset.

We need to separate those cases. Related: #2301

[dummdidumm]

True, there's a difference between an implicit update through a session update an an explicit user interaction.
Also related: #2252

[dummdidumm]

I adjusted the code so that only the implicit $session update no longer initiates a rerender and added a test. I also fixed another test which I believe tested the wrong expectation (page store updating on session change, which it shouldn't). Maybe this can also be fixed differently while also fixing #5247 by somehow not telling Svelte to update specific components, will look into that. Edit: Fixing that while removing the code here will bring back the (IMO) wrong page store behavior, so I'm creating another PR for this.