useCountdown

Requirements

• Export useCountdown as named and default exports. Accept a non-negative integer initialCount and a positive finite interval in milliseconds, default 1000. Inputs are guaranteed valid.
• Return count, isRunning, and stable start, stop, and reset functions. Initially count equals initialCount and isRunning is false.
• Start begins decrementing after one full interval. Repeated start calls while running do not restart the interval or create duplicate timers.
• Stop pauses and preserves the integer count. Starting again waits a fresh full interval; preserving a partially elapsed interval is not required.
• At zero stop automatically and never go negative. Starting at zero does nothing. Reset stops and restores the initialCount captured on the first render, even if that prop later changes.
• Changing interval while running preserves count and starts a fresh interval with the new duration. Derive elapsed ticks from Date.now so delayed callbacks catch up.
• Clear timers on pause, reset, unmount, and Strict Mode cleanup. The usage scaffold is provided; no HTML or CSS authoring is required.

Examples

Input: Start useCountdown(3), then wait 1000ms.
Output: count = 2, isRunning = true.

Input: Pause, wait, then reset.
Output: Pausing preserves count; reset restores 3 and stops.