GPU Credit Ledger With Expiry

Process a ledger of compute-credit events in order and return one result per spend and balance event, in order.

• ["grant", t, amount, expires_at] adds amount credits, usable at times t' with t <= t' < expires_at.
• ["spend", t, amount] takes amount credits from the unexpired grants, the grant with the earliest expires_at first. If the unexpired credits total less than amount, nothing is taken and the result is "insufficient"; otherwise "ok".
• ["balance", t] returns the unexpired credits remaining, as an integer.

Event times never decrease.