Implement Phase 5: SVG, JPG, PDF rendering

- SVG: strokes → SVG path elements with dashed/arrow support,
  coordinates scaled from 300 DPI to 72 DPI
- JPG: rasterization at 300 DPI using Go image package, Bresenham
  line drawing with round pen circles
- PDF: minimal PDF generation with raw operators, no external library
- 6 tests: SVG output, dashed style, arrow heads, JPG magic bytes,
  PDF header, page size calculations

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/render/render_test.go

@@ -0,0 +1,132 @@
+package render
+
+import (
+	"encoding/binary"
+	"math"
+	"strings"
+	"testing"
+)
+
+func makePointData(points ...float32) []byte {
+	data := make([]byte, len(points)*4)
+	for i, p := range points {
+		binary.LittleEndian.PutUint32(data[i*4:], math.Float32bits(p))
+	}
+	return data
+}
+
+func TestRenderSVG(t *testing.T) {
+	strokes := []Stroke{
+		{
+			PenSize:   4.49,
+			Color:     -16777216, // 0xFF000000 (black)
+			Style:     "plain",
+			PointData: makePointData(100, 200, 300, 400),
+		},
+	}
+
+	svg := RenderSVG("REGULAR", strokes)
+
+	if !strings.Contains(svg, "<svg") {
+		t.Fatal("expected SVG element")
+	}
+	if !strings.Contains(svg, "viewBox") {
+		t.Fatal("expected viewBox")
+	}
+	if !strings.Contains(svg, "<path") {
+		t.Fatal("expected path element")
+	}
+	if !strings.Contains(svg, `stroke="black"`) {
+		t.Fatal("expected black stroke")
+	}
+}
+
+func TestRenderSVGDashed(t *testing.T) {
+	strokes := []Stroke{
+		{
+			PenSize:   4.49,
+			Color:     -16777216,
+			Style:     "dashed",
+			PointData: makePointData(100, 200, 300, 400),
+		},
+	}
+
+	svg := RenderSVG("REGULAR", strokes)
+	if !strings.Contains(svg, "stroke-dasharray") {
+		t.Fatal("expected stroke-dasharray for dashed line")
+	}
+}
+
+func TestRenderSVGArrow(t *testing.T) {
+	strokes := []Stroke{
+		{
+			PenSize:   4.49,
+			Color:     -16777216,
+			Style:     "arrow",
+			PointData: makePointData(100, 200, 500, 200),
+		},
+	}
+
+	svg := RenderSVG("REGULAR", strokes)
+	if !strings.Contains(svg, "<line") {
+		t.Fatal("expected arrow head lines")
+	}
+}
+
+func TestRenderJPG(t *testing.T) {
+	strokes := []Stroke{
+		{
+			PenSize:   4.49,
+			Color:     -16777216,
+			Style:     "plain",
+			PointData: makePointData(100, 200, 300, 400),
+		},
+	}
+
+	data, err := RenderJPG("REGULAR", strokes, 90)
+	if err != nil {
+		t.Fatalf("render: %v", err)
+	}
+	if len(data) == 0 {
+		t.Fatal("expected non-empty JPG")
+	}
+	// Check JPEG magic bytes
+	if data[0] != 0xFF || data[1] != 0xD8 {
+		t.Fatal("expected JPEG magic bytes")
+	}
+}
+
+func TestRenderPDF(t *testing.T) {
+	pages := []Page{
+		{
+			PageNumber: 1,
+			Strokes: []Stroke{
+				{
+					PenSize:   4.49,
+					Color:     -16777216,
+					Style:     "plain",
+					PointData: makePointData(100, 200, 300, 400),
+				},
+			},
+		},
+	}
+
+	data := RenderPDF("REGULAR", pages)
+	if len(data) == 0 {
+		t.Fatal("expected non-empty PDF")
+	}
+	if !strings.HasPrefix(string(data), "%PDF") {
+		t.Fatal("expected PDF header")
+	}
+}
+
+func TestPageSizePt(t *testing.T) {
+	w, h := PageSizePt("REGULAR")
+	if int(w) != 612 || int(h) != 792 {
+		t.Fatalf("REGULAR: got %v x %v, want 612 x 792", w, h)
+	}
+	w, h = PageSizePt("LARGE")
+	if int(w) != 792 || int(h) != 1224 {
+		t.Fatalf("LARGE: got %v x %v, want 792 x 1224", w, h)
+	}
+}