Checkpoint
One last thing before we move on. Same surface as a write step — but the lesson doesn't complete until this passes.
Add the causal mask — the decoder rule that keeps next-token training from cheating. For each token i of 3, compute scaled dot-product scores against ONLY keys j <= i, softmax over those, and pad the row with 0.0 up to length 3. Print f"token {i}: {row}" (weights rounded to 2) and finish with the literal line "upper triangle is zero: no token sees the future".