Events()/EventsAfter() previously closed the channel silently when the
16-slot buffer overflowed, indistinguishable from a graceful server end.
Reserve one slot and emit a terminal EventResult wrapping the new exported
ErrSlowConsumer so overflow is always observable. The blocking
SubscribeEvents path (gRPC flow-controlled) is unchanged.
archreview: CLI-01 (P0). Verified: gofmt clean, go build ./..., go test ./... (+ -race) pass.
Joseph Doherty