Make an accidentally quadratic rollup meet its two second budget

The payments rollup aggregates a day of ledger rows into one summary per account. It finds the slot for each row by scanning a list of accounts seen so far, so the work grows with rows times accounts: 200,000 rows over 6,000 accounts takes about twelve seconds against a two second budget. The aggregation is correct, and the visible tests pass, so this is a complexity problem rather than a logic problem. The fix is a single pass with a hashed lookup. The output contract, including the fact that accounts come out in the order they first appear, has to survive it, and the hidden tests time a second ledger of a different shape so that making the shipped fixture fast is not enough.