feat(ioscompanion): add Describe to companion transport

internal/driver/ioscompanion/transport/grpc.go

@@ -47,6 +47,26 @@ func (c *grpcCompanion) AccessibilityInfo(ctx context.Context) (string, error) {
 	return resp.GetJson(), nil
 }
 
+func (c *grpcCompanion) Describe(ctx context.Context) (ScreenDescription, error) {
+	resp, err := c.client.Describe(ctx, &pb.TargetDescriptionRequest{})
+	if err != nil {
+		return ScreenDescription{}, err
+	}
+	return screenDescriptionFrom(resp), nil
+}
+
+// screenDescriptionFrom extracts the point dimensions and scale from a describe
+// response. The generated getters are nil-safe, so a response missing the
+// nested messages yields a zero-valued ScreenDescription rather than panicking.
+func screenDescriptionFrom(resp *pb.TargetDescriptionResponse) ScreenDescription {
+	dimensions := resp.GetTargetDescription().GetScreenDimensions()
+	return ScreenDescription{
+		WidthPoints:  int(dimensions.GetWidthPoints()),
+		HeightPoints: int(dimensions.GetHeightPoints()),
+		Scale:        dimensions.GetDensity(),
+	}
+}
+
 func (c *grpcCompanion) SendHID(ctx context.Context, events ...HIDEvent) error {
 	stream, err := c.client.Hid(ctx)
 	if err != nil {

internal/driver/ioscompanion/transport/grpc_test.go

@@ -42,6 +42,31 @@ func TestProcessStateFromProto(t *testing.T) {
 	}
 }
 
+func TestScreenDescriptionFrom(t *testing.T) {
+	resp := &pb.TargetDescriptionResponse{
+		TargetDescription: &pb.TargetDescription{
+			ScreenDimensions: &pb.ScreenDimensions{
+				Width:        828,
+				Height:       1792,
+				Density:      2,
+				WidthPoints:  414,
+				HeightPoints: 896,
+			},
+		},
+	}
+	got := screenDescriptionFrom(resp)
+	want := ScreenDescription{WidthPoints: 414, HeightPoints: 896, Scale: 2}
+	if got != want {
+		t.Errorf("screenDescriptionFrom = %+v, want %+v", got, want)
+	}
+}
+
+func TestScreenDescriptionFromNilSafe(t *testing.T) {
+	if got := screenDescriptionFrom(&pb.TargetDescriptionResponse{}); got != (ScreenDescription{}) {
+		t.Errorf("screenDescriptionFrom(empty) = %+v, want zero", got)
+	}
+}
+
 func TestTarGzipDirectory(t *testing.T) {
 	root := t.TempDir()
 	bundle := filepath.Join(root, "Sample.app")

internal/driver/ioscompanion/transport/transport.go

@@ -16,6 +16,10 @@ type Companion interface {
 	// raw flat-format JSON string the companion emits.
 	AccessibilityInfo(ctx context.Context) (string, error)
 
+	// Describe reports the target's screen dimensions in points, along with
+	// the pixel scale when the companion supplies one.
+	Describe(ctx context.Context) (ScreenDescription, error)
+
 	// SendHID opens the HID stream, sends every event in order, then closes.
 	SendHID(ctx context.Context, events ...HIDEvent) error
 
@@ -43,6 +47,16 @@ type Companion interface {
 	Close() error
 }
 
+// ScreenDescription carries the target screen geometry. Width and Height are
+// in points (the coordinate space HID events and the accessibility frames use).
+// Scale is the pixel-per-point density, or 0 when the companion did not report
+// one.
+type ScreenDescription struct {
+	WidthPoints  int
+	HeightPoints int
+	Scale        float64
+}
+
 // ProcessState mirrors the companion's notion of whether an app is running.
 type ProcessState int