[5.43.12] `unshift` breaks keyed `#each` index

[henrykrinkle01]

Describe the bug

When using unshift to add elements to an array used in a keyed #each block, the index doesn't correctly update, breaking everything inside that depends on it (element id, input names, etc).

Reproduction

Working:
https://svelte.dev/playground/2c88b99bca074875bc50c2b9d0462545?version=5.43.11
Not working:
https://svelte.dev/playground/2c88b99bca074875bc50c2b9d0462545?version=5.43.12

Logs

System Info

svelte@5.43.12

Severity

blocking an upgrade

[henrykrinkle01]

@Rich-Harris Sorry for ping but I think #17191 is the culprit. Every operation that adds an element to an array at the beginning or middle (unshift, splice, rest operator) doesn't correctly update the index. I don't expect an immediate fix but could this be flagged as a bug?