Investigate module storage handling in block state

**Task description**

It appears that module sources are stored in the block state as soon as they are added in blocks, and not just on finalization. This could lead to unnecessary or redundant storage in the case of blocks that are never finalized, and so this potentially problematic behaviour. We should investigate whether the present behaviour is acceptable, and find a fix if necessary.

Start by tracing where module sources are stored when blocks are added and how that differs from finalization. Determine whether unfinalized blocks retain redundant module data, then document the acceptable behavior or identify the required fix and its verification criteria.

Written by the indexing model from the issue text.