chore: only call mark_reactions when updating a source

[Rich-Harris]

This implements something I've been wanting to get at for a while. Currently, we call mark_reactions in two places — when updating a source, and when updating a derived. The latter doesn't really make sense — we should only mark reactions when sources update.

This fixes that, and allows us to simplify things a bit — there's no more force_schedule. It also allows us to move mark_reactions (and inspect_effects) out of the neverending module that is runtime.js and into sources.js.

It works by using version tracking as the universal mechanism for knowing if a reaction is dirty. We can use current_version for effects, rather than increment_version(), so this doesn't increase the (infinitesimal) danger of an overflow. (I thought I'd be able to use the same trick for deriveds, but it doesn't work for some reason — will continue investigating.)

Performance-wise it's basically neutral, except for kairo_mux_unowned which is slower. I'm not exactly sure why but my hunch is that it's somehow related to the fact that we're pushing reactions further down — it feels weird that we do that there in addition to doing it inside update_reaction. Feels off, will look into it. With the most recent commits, this is significantly faster than main. I'm still a bit confused by the is_unowned stuff inside check_dirtiness, but we can worry about that another time.

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