fix: diff outputs for queries participating in a cycle

[MichaReiser]

The title is a bit misleading but it's my intention to have a fix. For now, this only adds a regression test.

The problematic scenario is a query participating in a cycle (not the cycle head)
that creates some tracked structs only in later iterations. The issue here is that
we run diff_outputs after each iteration, which will delete any tracked struct
that hasn't been created in the first iteration... However, other queries might depend
on those tracked structs already because salsa called maybe_changed_after on them.

The fix is that we only call diff_outputs once the value is finalized, the same as for cycle heads.
What makes this difficult is that finalization happens lazily.

[netlify]

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[codspeed-hq]

CodSpeed Performance Report

Merging #980 will not alter performance

_{Comparing MichaReiser:tracked-deleted (1290c58) with master (52a4057)}

Summary

✅ 12 untouched benchmarks