checkpoint: make Write(WriteRequest) the only public persistent write

The committed write operations are now internal dispatch targets of Write(ctx, WriteRequest); callers use the request union exclusively: WriteCommitted -> writeSession (Write WriteSession{...}) UpdateCommitted -> backfillTranscript (Write BackfillTranscript{...}) UpdateSummary -> backfillSummary (Write BackfillSummary{...}) UpdateCheckpointSummary -> backfillAttribution (Write BackfillAttribution{...})

Production already wrote through the union; this migrates the test callers and unexports the dispatch targets, so 'Committed' is gone from the persistent write surface and Write is the single entry point. No behavior change.