fix: reconnect disconnect deriveds after batch processing

[Rich-Harris]

WIP. This is a different take on #17095 — rather than change is_dirty, which is already a slightly confusing chunk of code, what if we reconnect deriveds after batches are processed? I worry that calling a function called is_dirty purely for its side-effects will make things harder to follow in future. We're already breaking the rules by having side-effects in is_dirty at all, and I would love to find a way to clean that up (other than setting MAYBE_DIRTY signals to CLEAN, which feels okay).

On that: I'm pretty sure we're using terminology incorrectly at present. A derived is UNOWNED if it is created outside an active reaction, and as such that flag should never be toggled, unlike DISCONNECTED which describes the current state of a derived. I feel like this corner of the codebase is overdue for an inspection.

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Playground

pnpm add https://pkg.pr.new/svelte@17102

[dummdidumm]

While this also fixes the issue I don't believe we can do this - the derived could be unowned for a reason, and then you'd leak memory but just blindly reconnecting them. Imagine I only did let { double } = $props(); double; and not use double anywhere else. It would leak in that case.

[Rich-Harris]

superseded by #17105