The issue was that the forward compatibility writing code for channel groups in
layered actions was building a temporary legacy listbase, but was not clearing
it properly afterwards. This was then getting caught by an assert that ensured
that layered-action groups didn't have legacy data in them.
The reason the listbase wasn't getting cleared properly is because the
prev/next listbase pointers were getting cleared using a `LISTBASE_FOREACH`
loop, and thus the loop never progressed past the first item.
Additionally, this mistake wasn't just in the channel groups writing code, but
also the forward compatibility writing code for fcurves.
This fixes the issue in both places by switching the loops to
use`LISTBASE_FOREACH_MUTABLE`.
Based on discussion with @dr.sybren, this also removes the assert that caught
the issue. The situation it guards against is actually completely benign, and
the existence of the assert is contrary to the comments in the forward-compat
writing code explaining why its approach is okay.
Pull Request: https://projects.blender.org/blender/blender/pulls/126970
515843ff3a