fix: adjust effect ordering

[dummdidumm]

This started out investigating a bug on the current preview site. When you navigate to the docs, then to something else than the introduction, then back to the introduction, you get an error. The culprit is the pre effects getting flushed by some render effect. We can fix this by adding a "is this legacy mode" check (which I removed in #10736; turns out we still need it). But then I realized that the whole logic is a bit flawed either way: Flushing render effects directly after pre effects and vice versa can get things out of order in runes mode, too. I've added a failing test in this PR. I don't have a good idea yet how to properly fix this, but I think it involves getting rid of the "eagerly flush this" and instead smartly inserting the render/pre effects in the right order, which means somehow honoring the currently flushed render/pre-effects when queuing new effects.

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[dummdidumm]

closing in favor of #10783