docs(diagrams): refresh ai-optimized mermaid flows after audit

docs/api/PROTOCOL.md

@@ -57,6 +57,11 @@ sequenceDiagram
     LQ-->>G: starts work immediately
   else lane busy
     Note over LQ: work queued (FIFO) for this lane
+    opt mode=interrupt and newer request arrives
+      LQ->>SB: request cancellation of active lane run
+      SB->>A: cancel() (best effort)
+      G-->>C: transient preempt notice (queue.preempt)
+    end
   end
 
   G->>A: process(message) in that session