Diff session turns

Structured per-file diffs of the files a turn changed. A turn runs from the first prompt after the session was last idle until its next idle marker, so prompts steered in while it was busy belong to the same turn; to extends the range through a later turn. Compares the range's first recorded snapshot with its last; a step still running in the active session compares against the working copy. Ranges that span a location change are rejected. In sessions without any idle marker, a prompt's turn spans until the next user message.