Allow File Data Sources to update an entity

Currently, different File Data Sources can create multiple entities with the same ID.

File Data Sources should instead be able to update an entity with the same ID. Note that this will not be an upsert, which is the current pattern that is used for chain-based data sources. Instead this should completely overwrite the prior entity.

Situation:

file data source A created in block 1
A is found, saves entity X
file data source B created in block 5
file data source C created in block 10
C is found, saves entity X again <-- this is the most recent
B is found, saves entity X again

Entity updates should apply a "most recent wins" approach, where the time is determined by block time, not handler execution time. This is a new pattern (where entities might be created with closed block ranges), and order will need to be resolved within blocks (as well as between blocks)

No file, test, or entry point is named. Start by tracing how file data sources save entities and compare that path with the existing chain-based upsert pattern, then determine how block time and ordering within a block are represented. Done means repeated saves use the most recent block-ordered value and completely overwrite the prior entity without upserting.