feat(trace): add WriteHTML + Step.HTMLAvailable

Per-step HTML lives in <run>/html/step-NNNNN.html so trace.jsonl stays
line-greppable on apps with hundreds-of-KB DOMs.

internal/trace/writer_test.go

@@ -269,6 +269,65 @@ func TestWriteAfterClose_Errors(t *testing.T) {
 	}
 }
 
+func TestWriteHTML_WritesToHTMLDir(t *testing.T) {
+	directory := t.TempDir()
+	writer, _ := NewWriter(directory)
+	defer writer.Close()
+
+	html := []byte("<!doctype html><html><body>hi</body></html>")
+	if err := writer.WriteHTML(3, html); err != nil {
+		t.Fatal(err)
+	}
+	if err := writer.WriteHTMLAfter(3, html); err != nil {
+		t.Fatal(err)
+	}
+
+	got, err := os.ReadFile(filepath.Join(directory, "html", "step-00003.html"))
+	if err != nil {
+		t.Fatalf("step-00003 missing: %v", err)
+	}
+	if string(got) != string(html) {
+		t.Errorf("html bytes wrong: %q", got)
+	}
+	if _, err := os.Stat(filepath.Join(directory, "html", "step-00003-after.html")); err != nil {
+		t.Errorf("step-00003-after missing: %v", err)
+	}
+}
+
+func TestWriteHTML_EmptyByteSliceIsNoop(t *testing.T) {
+	directory := t.TempDir()
+	writer, _ := NewWriter(directory)
+	defer writer.Close()
+
+	if err := writer.WriteHTML(1, nil); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := os.Stat(filepath.Join(directory, "html")); !os.IsNotExist(err) {
+		t.Errorf("html dir should not exist after empty write")
+	}
+}
+
+func TestStep_HTMLAvailableRoundTrip(t *testing.T) {
+	directory := t.TempDir()
+	writer, _ := NewWriter(directory)
+	defer writer.Close()
+
+	if err := writer.WriteStep(Step{Index: 1, HTMLAvailable: true}); err != nil {
+		t.Fatal(err)
+	}
+	if err := writer.WriteStep(Step{Index: 2}); err != nil {
+		t.Fatal(err)
+	}
+	body, _ := os.ReadFile(filepath.Join(directory, "trace.jsonl"))
+	lines := strings.Split(strings.TrimSpace(string(body)), "\n")
+	if !strings.Contains(lines[0], `"html_available":true`) {
+		t.Errorf("html_available missing in step 1: %s", lines[0])
+	}
+	if strings.Contains(lines[1], "html_available") {
+		t.Errorf("html_available should be omitted when false: %s", lines[1])
+	}
+}
+
 func TestNewWriter_CreatesNestedDirectory(t *testing.T) {
 	parent := t.TempDir()
 	target := filepath.Join(parent, "runs", "2026-04-17T22-30-00")