fix: use local mutable sources for props in legacy mode in case they are indirectly invalidated

[Rich-Harris]

This is a real strange one. There's a test, binding-input-text-contextual-reactive, that passes on main but doesn't work in a real app (try editing the items — the summary never updates).

The diff below fixes it, but I cannot for the life of me make the test fail without it.

Update okay I managed to add a new test that fails on main. The new code makes more sense, I'm surprised the bug remained hidden as long as it did.

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🦋 Changeset detected

Latest commit: 4febc9e

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

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svelte	Patch

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Playground

pnpm add https://pkg.pr.new/svelte@16038