Allow requesters to cancel unaccepted task results #202

Host-lifecycle prerequisite for #198, following #199. Independent of the offline harness PR #201.

Current task state transitions allow result-ready -> accepted only, leaving a returned result impossible to cancel. Permit the existing requester-only task-cancel action from result-ready, preserving the returned summary and the actor trail.

Acceptance:

• The authenticated requester can cancel an unaccepted returned result.
• Another owner, a forged body actor, an anonymous/revoked session cannot cancel it.
• Cancellation persists after restart, preserves the result for audit, and later acceptance/return/start cannot revive the task.
• Accepted, failed and cancelled tasks remain terminal; existing approval holds and grant behavior remain unchanged.
• Document cancellation as task disposition, not confirmation that an external runtime has stopped.

No new endpoint, runtime privilege, social friendship exception or team-mode enablement.