SessionExecution activeClaims boundary races

Trap: registering activeClaims only after acquisition, or treating every registered claim as durable until finalizer cleanup, looks consistent but exposes both commit-to-registration and durable-clear-to-unregistration races. Fix: pre-register behind an acquisition latch, retain registration through drain finalizers, and reconcile SessionExecutionClaim.Lost during pre-interrupt transition as already settled when successful execution already deleted the claim; interruption must neither strand an owner nor surface HTTP 500.