Checkpoint
One last thing before we move on. Same surface as a write step — but the lesson doesn't complete until this passes.
Checkpoint — the mission in miniature. Write pipeline(raw) that flattens/casts each record (user from profile, float amount), drops-and-counts failures, and returns (clean_rows, report) where report is {"kept": n, "dropped": n}. Print the report and the per-user total for maya.