Fix the replay of a guarded pay button

Your checkout button guarded against double-submit with a React state flag, and fast double-clicks still charged the card twice. The fix is a ref used as a lock, plus an idempotency key the server checks. An idempotency key is an id the server uses to spot a repeated request. The team wants a replay that shows which clicks become real charges.

An assistant wrote settled_charge_indices(click_times, keys, charge_ms). It is in the editor. It has a bug. Find it and fix it.

Input and output:

• click_times: when each click arrived, in milliseconds, in order. Times are 0 or more and never go down.
• keys: the idempotency key sent with each click. keys[i] belongs to click i.
• charge_ms: how long one request to the server takes, in milliseconds.
• Return the indices (from 0) of the clicks that charge the card, in increasing order.

Rules the tests check:

• Use only the first n clicks, where n is the length of the shorter list. Ignore the extra entries of the longer one.
• A negative charge_ms counts as 0.
• The first click always passes the lock. A later click passes when its time is at or after the release time: the time of the last click that passed, plus charge_ms. A click before that is refused. It sends nothing and changes nothing.
• Every click that passes sends a request and takes the lock, whether or not its key is new. The release time becomes its time plus charge_ms.
• If an earlier charge used the same key, the server replays the stored result and nothing is charged. Otherwise the click charges the card and its key is recorded.
• Keys compare exactly. Case matters, and the empty string is a key like any other.
• No clicks returns an empty list.