New turn_preempted event lets a rapid follow-up message interrupt a running turn

What

• A new system/turn_preempted event (reason rapid_followup) is sent when Claude Code itself stops a running turn because the person sent a follow-up message before the turn produced any output; any running shell commands are backgrounded rather than killed, and the follow-up is answered right away instead of waiting in queue.
• This only happens when the connected client has declared support for it: a new SDK capability, rapidFollowupPreempt (declared via declareRapidFollowupPreempt, read back as sdkRapidFollowupPreempt()), mirrors the existing perTaskStopAffordance capability and tells Claude Code the client knows how to handle a turn_preempted event.
• It is also gated behind a rollout flag (tengu_zippy_spindle), off by default, so both the client declaration and the flag must be enabled. Preemption is skipped for slash commands, bridge-originated messages, and similar cases where it wouldn't make sense.

Why Without this, sending a quick follow-up while Claude Code is still working on the previous message just queues it silently. With this enabled (and supported by the client), the follow-up can preempt the still-running turn so it gets addressed immediately.