Register an event. Internal API to be used by regions.

Reports that an Xcode build cache archive finished uploading, so backend can upsert the index row that advertises it. Sent by controller-manager at instance termination, on the same event path every other instance transition uses.

The upload is the publication; this row only says where the object is. A lost event therefore leaves a newer object that still validates against its own metadata, and the worst outcome is one cold build.