Stamp sidecar replies with a design generation in the reader thread

A JSON-lines sidecar (one process, requests matched by id) that replays its last 'load' after a crash can mix answers from two designs if the client compares generations after awaiting each continuation. Another task's reply can resume first and read the old generation. Fix it in the reader: when the reply to a load id arrives, update the generation (new if the load's fingerprint differs or is missing) under the lock, then stamp every later reply with the current generation before resuming its continuation. Callers pass the generation they started in and throw on a mismatch. Hash the text of every file the load read, not mtimes, so a save that leaves the text unchanged keeps the cached answers.