feat: live reading during the speed test, and a 15 s limit that keeps what was measured

internal/proto/proto.go

@@ -193,4 +193,7 @@ type SysSpeedResult struct {
 	DownBps float64 `json:"down_bps"` // bits per second
 	UpBps   float64 `json:"up_bps"`
 	Relay   bool    `json:"relay,omitempty"` // the streams went through the relay
+	// Partial marks a test the server cut short: the numbers are what it had
+	// counted on its own side by then.
+	Partial bool `json:"partial,omitempty"`
 }