refactor(capture): screen capture in subprocess

- Move screen capture to child process
- Isolate GDI/BitBlt crashes from agent
- Use -capture flag for internal helper mode
- Add protocol for frame request/response
- Make monitor enumeration thread-safe

lib/capture/protocol_test.go

@@ -0,0 +1,53 @@
+package capture
+
+import (
+	"bufio"
+	"bytes"
+	"testing"
+
+	"tea.chunkbyte.com/kato/go-worm/lib/models"
+)
+
+func TestCaptureProtocolRoundTrip(t *testing.T) {
+	t.Parallel()
+	var buf bytes.Buffer
+	if err := writeRequest(&buf, 1, "jpeg", 40); err != nil {
+		t.Fatal(err)
+	}
+	mon, format, quality, err := readRequest(bufio.NewReader(bytes.NewReader(buf.Bytes())))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if mon != 1 || format != "jpeg" || quality != 40 {
+		t.Fatalf("request = %d %s %d", mon, format, quality)
+	}
+
+	buf.Reset()
+	want := models.CapturedImage{
+		ContentType: "image/jpeg",
+		Data:        []byte{0xff, 0xd8, '\n', 0x00, 0xff},
+		Left:        10, Top: 20, Width: 100, Height: 50,
+	}
+	if err := writeFrame(&buf, want); err != nil {
+		t.Fatal(err)
+	}
+	got, err := readResponse(bufio.NewReader(&buf))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got.ContentType != want.ContentType || got.Left != want.Left || got.Width != want.Width || !bytes.Equal(got.Data, want.Data) {
+		t.Fatalf("frame mismatch: %#v", got)
+	}
+}
+
+func TestCaptureProtocolError(t *testing.T) {
+	t.Parallel()
+	var buf bytes.Buffer
+	if err := writeErr(&buf, "no displays found"); err != nil {
+		t.Fatal(err)
+	}
+	_, err := readResponse(bufio.NewReader(&buf))
+	if err == nil || err.Error() != "no displays found" {
+		t.Fatalf("err = %v", err)
+	}
+}