feat: add text annotation tool

internal/application/annotation_test.go

@@ -51,3 +51,45 @@ func TestApplyAnnotationsNoAnnotationsReturnsOriginalBytes(t *testing.T) {
 		t.Fatalf("expected original bytes")
 	}
 }
+
+func TestApplyAnnotationsDrawsTextIntoPNG(t *testing.T) {
+	src := image.NewRGBA(image.Rect(0, 0, 120, 80))
+	draw.Draw(src, src.Bounds(), &image.Uniform{C: color.White}, image.Point{}, draw.Src)
+
+	var buf bytes.Buffer
+	if err := png.Encode(&buf, src); err != nil {
+		t.Fatalf("encode source: %v", err)
+	}
+
+	out, err := ApplyAnnotations(buf.Bytes(), []Annotation{
+		{
+			Tool:  "text",
+			Color: "#ef4444",
+			Points: []Point{
+				{X: 10, Y: 10},
+			},
+			Text: "T",
+		},
+	}, 1)
+	if err != nil {
+		t.Fatalf("apply annotations: %v", err)
+	}
+
+	img, err := png.Decode(bytes.NewReader(out))
+	if err != nil {
+		t.Fatalf("decode output: %v", err)
+	}
+	found := false
+	for y := 8; y < 40 && !found; y++ {
+		for x := 8; x < 40; x++ {
+			got := color.RGBAModel.Convert(img.At(x, y)).(color.RGBA)
+			if got.R > 180 && got.G < 180 && got.B < 180 {
+				found = true
+				break
+			}
+		}
+	}
+	if !found {
+		t.Fatalf("expected red text pixels near annotation point")
+	}
+}