Harden concurrency assertions for Zarr writing

[normanrz]

We want to switch our default data format to zarr3. In contrast to wkw, zarr3 doesn't allow concurrent writes (as uncompressed wkw does) at all. Therefore, we should be more careful when writing partial shards. Perhaps, we should elevate some of the warnings to assertions to better enforce shard-aligned writing.

[hotzenklotz]

After discussing this, we figured it is very hard to do in the libs. We don't have a support good idea yet. Likely, this needs to be checked and asserted in VX.

This issue becomes important for once we switch to Zarr3 as the default dataset format.