fix: don't transform reassigned state in labeled statement in $derived

[paoloricciuti]

Closes #15719

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Playground

pnpm add https://pkg.pr.new/svelte@15725

[Rich-Harris]

made a couple small drive-by tweaks, mostly relating to the code that was already there:

• better to coerce types upstream than downstream, so we don't need optional chaining etc
• in general we should prefer array.every(foo) to !array.some(bar) for the sake of those of us with brains that struggle to parse double negatives — 'every member of this array satisfies this predicate' is less of a tangle than 'it is not the case that one or more members of this array satisfies this predicate'