Checkpoint
One last thing before we move on. Same surface as a write step — but the lesson doesn't complete until this passes.
Build the full checkpoint bundle. descend(w, v, start_step, steps, lr=0.1, m=0.9) runs momentum descent on w² and returns the complete state dict {"w", "v", "step", "config": {"lr", "m"}}. Run 8 steps straight through; then run 4, checkpoint, and resume 4 more FROM the bundle. Print both end states (f"full: step={...} w={...:.6f}" / f"resumed: step={...} w={...:.6f}") and "identical:" comparing the entire dicts.