Fix cell modifications causing cell output reload and shift to active cell index

[krassowski]

• Add a failing test case for granular modifications
• Fix the issue by granularly updating the cells

References:

• this is a follow-up to Improve reloading of documents to avoid spurious side-effects #355
• this fixes the case of a full cell reload if its source gets changed, which lead to active cell being off afterwards, as noted in Notebook jumps to the first cell on touch (out of band changes) jupyterlab/jupyterlab#18100 (comment)
• the tests could be more explicit if event classes were not empty, Event classes are empty y-crdt/pycrdt#316

Before

Screencast.From.2025-11-20.15-04-12.mp4

After

Screencast.From.2025-11-20.15-02-43.mp4

[davidbrochart]

Maybe just pass old_ycell, since old_cell can be created from it?

[krassowski]

I thought about doing that, but we are still using old_cell in line 270 below

[davidbrochart]

Thanks.

[krassowski]

I will release this as v3.3.0 along with #359 - minor bump since the other PR increased required Python version to 3.10.