[architecture] Fix October broken sessions root causes

_Automatically composed from the parent epic at dispatch time._

7 independent children: C1 kill fence on slot generation; C2 relaunch waits on node-reported predecessor gone; C3 vanished-incident evidence; C4 launch_failed skip + cross-account refund; C5 stalled detector (+VM); C6 lane forge-auth blocker; C7 classifier fixes. Memory: none applied.