Implement formatDuration(ms) with strict spec

Create src/fmt.js exporting formatDuration(ms) (integer ms, non-negative). Output: less than 1s → "Ns" (e.g. 500 → "0s", no decimals); less than 60s → "Ns"; less than 60min → "Nm Ss"; else → "Hh Mm". Always zero-pad S and M to two digits only when there is also an H part. The tests in test.js pin every edge.