Wire playlist into main.go — one case line and one line of usage text.

$ algo gen -n 100000 -seed 42 -o big.jsonl
wrote 100000 items to big.jsonl (seed=42, order=shuffled, dup-rate=0.00)

$ algo playlist -in big.jsonl -from 47 -to 3

Step 4 looks at what it printed. Before you read on — write down your guess: which structure wins when moving from 47 to 3?

The list's move is O(1) and the array's is O(n). The answer looks obvious.