Stop the session's in-flight turn

The explicit Stop (kernel work model, D5). Cancels the running turn promptly: the stream emits the terminal turn-ended frame with reason cancelled, partial assistant text is committed to the transcript, and the session survives for the next turn — stopping a turn does not cost the conversation. Idempotent: cancelling when no turn is running is a 200 no-op, not a conflict. Disconnect inference backs this endpoint up: a turn whose SSE watchers all vanish is cancelled after a short grace window, while a turn that never had a watcher (fire-and-forget) runs to its budget.