fix: memoize useFetchers

[jacobparis]

useFetchers currently returns a new array on every render. This PR memoizes it based on the internal useDataRouterState which appears to be stable as long as the fetchers aren't actually changing.

The included test fails before the fix with fetchers count: 3 and passes after the fix

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[brophdawg11]

Thanks for this PR - I forgot to go back and look at the implementation. I don't actually think this was stable before - it was always a new array instance .

I agree it probably should be but I need to take a bit of a closer look to ensure that we're guaranteeing a brand new Map on state.fetchers every time we update a single fetcher 👍

[jacobparis]

Admittedly I didn't dig through the history to see if it used to be

Instead my expected behaviour was based on this tweet from @ryanflorence https://twitter.com/ryanflorence/status/1746281296475943243?s=46&t=9QWqc33eA_iECT8wUUL0VA

[brophdawg11]

Ah - yeah that tweet's referring to useFetcher - which is correctly memoized. useFetchers probably should be too though - I was just checking to see if this was a regression or just something we hadn't yet tackled 👍

[brophdawg11]

At a quick glance, I think we always do a new Map when we set fetchers - but I'm slightly concerned that we mutate in place to update fetchers prior to setting the new Map. I think I'd like to update the router to always do the new Map up front and then mutate and commit it - so that we never mutate state.fetchers in the first place. I'll try to tackle that soon 👍