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 lab end-to-end. Write feature_table(db, cutoff) returning (user, n_valid, total) per user (every user, cutoff-respecting, NULL-safe, ?-parameterized, sorted by user) AND gate(rows) returning False if any total is negative or the table is empty. Print the rows, then the gate verdict.