Error rate alert trigger

A gateway tags each request with an error flag, giving you a list of 0s and 1s in arrival order (1 = failed request). The alerting rule watches the last `window` requests: it trips at the first position where the most recent `window` flags contain at least k ones. Return the smallest index i (0-based, counting from the window-th request, so i >= window - 1) where flags[i-window+1..i] holds k or more ones; return -1 if the rule never trips (including when fewer than `window` requests exist). Assume window >= 1 and k >= 1.