agent-end was worker-authored only, so a start already forwarded to
observers lost its paired end whenever the worker could no longer speak
- the grace force-settle terminating a wedged script, or an unexpected
worker death - stranding progress consumers with agents that never
finish (ds-review-bot finding on #233).
The host now keeps a ledger of forwarded starts and funnels every
agent-end through one gate: worker-reported ends pair (and clear) their
entry, and both termination paths drain the remainder as synthesized
'cancelled' ends BEFORE the run settles, so ends always precede
workflow/end. A real settlement racing the force-settle loses to the
synthesized cancellation - the same first-wins override onResult applies
to the run's own result.