fix: only re-run directly applied attachment if it changed

[Rich-Harris]

alternative to #15948 — instead of warning that it might not work as expected, we can just make it work as expected.

This doesn't address the similar issue for spread attributes (#15949, #15961) but it seems like a no-brainer for direct attachments on elements.

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[changeset-bot]

🦋 Changeset detected

Latest commit: 2b13874

The changes in this PR will be included in the next version bump.

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15962-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15962

[Rich-Harris]

errrr i should have run the tests before pushing this i guess

[paoloricciuti]

Makes absolute sense...I thought it was "reasonable" to rerun in legacy mode since the whole object was invalidated but since attachments are brand new and this is the expected behavior it's way better