fix: make each items reassignable

[Rich-Harris]

Closes #12254. I thought this would be a super simple fix, and it is, but in the process of adding a test I uncovered a separate bug: if a bound prop is updated during rendering, it isn't invalidated. See Svelte 4 vs Svelte 5. I don't have any immediate thoughts on how to fix that, so I'm just going to leave this here for now.

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