fix(CLI-01): Go Events() emits terminal ErrSlowConsumer on overflow

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.

clients/go/mxgateway/errors.go

@@ -1,11 +1,19 @@
 package mxgateway
 
 import (
+	"errors"
 	"fmt"
 
 	pb "gitea.dohertylan.com/dohertj2/mxaccessgw/clients/go/internal/generated"
 )
 
+// ErrSlowConsumer is the terminal error sent on the Events/EventsAfter
+// (cancel-when-full) path when the buffered results channel overflows because
+// the consumer fell behind. It is delivered as the final EventResult.Err before
+// the channel closes, so overflow is always observable rather than silently
+// dropping events. Match it with errors.Is.
+var ErrSlowConsumer = errors.New("mxgateway: event consumer fell behind; stream terminated")
+
 // GatewayError wraps transport-level gRPC failures.
 type GatewayError struct {
 	// Op names the operation that failed (for example "dial" or "invoke").