Fix the poll schedule for a courier map

An AI kept a delivery map fresh by polling the courier's location every 200 ms in a while true loop. It never stopped, kept polling in the background, and drained the battery. The replacement uses a timer with separate foreground and background intervals, and cancels it when the map goes away. Your team checks the schedule it produces.

An assistant wrote plan_poll_times(state_changes, foreground_interval_ms, background_interval_ms, horizon_ms), the schedule. It is in the editor. It has a bug. Find it and fix it.

Input and output:

• state_changes: the app's state over time, each written ms:state, in increasing time order. The states are foreground, background and stopped.
• foreground_interval_ms and background_interval_ms: the polling interval in each state.
• horizon_ms: the last moment to plan for, included.
• Return every time a poll request goes out, in increasing order.

Rules the tests check:

• Split a line at its first :. Skip it unless the time is one or more digits from 0 to 9 and the state is one of the three above.
• Each kept line starts a segment that lasts until the next kept line's time. The last segment runs past horizon_ms.
• In a foreground or background segment, a poll fires at the segment's start time and then once every interval, as long as the time is before the segment's end and not after horizon_ms.
• An interval of 0 or less means no polls in that state.
• stopped cancels the timer for good: no poll fires at or after it, even if a later line says foreground or background.