Session Stats Calculator

You are given a JSON array of raw event objects, each containing a user_id (string), a timestamp (integer Unix seconds), and an event_type (string). A session for a user is defined as a sequence of events whose consecutive timestamps are no more than a given timeout threshold apart. Your task is to write a function that, for each user, groups their events into sessions and then returns a JSON object mapping every user_id to an array of session statistics. Each session statistic is an object with keys "start", "end", "duration", and "event_count". "start" is the timestamp of the first event in the session, "end" is the timestamp of the last event, "duration" is end-start in seconds, and "event_count" is the number of events in that session. Sessions must be reported in chronological order. If a user has no events, the mapping should still include the user_id with an empty array.