feat(driver): native DoubleTap RPC for a tight inter-tap gap

Composing two Tap round trips from the Go client spread the taps by
hundreds of milliseconds on iOS, wide enough for the app to navigate
between them, so double-submission races could never reproduce. The
sidecar now lands both taps back-to-back next to the device transport.

proto/driverpb/driver_test.go

@@ -16,6 +16,7 @@ func TestDriverServiceDescriptor(t *testing.T) {
 		"Launch":      true,
 		"Terminate":   true,
 		"Tap":         true,
+		"DoubleTap":   true,
 		"TapSelector": true,
 		"InputText":   true,
 		"EraseText":   true,