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.go

@@ -0,0 +1,98 @@
+package capture
+
+import (
+	"bufio"
+	"errors"
+	"fmt"
+	"io"
+	"strconv"
+	"strings"
+
+	"tea.chunkbyte.com/kato/go-worm/lib/config"
+	"tea.chunkbyte.com/kato/go-worm/lib/models"
+)
+
+var ErrMonitorNotFound = errors.New("monitor not found")
+
+func writeRequest(w io.Writer, monitor int, format string, quality int) error {
+	_, err := fmt.Fprintf(w, "C %d %s %d\n", monitor, format, quality)
+	return err
+}
+
+func readRequest(r *bufio.Reader) (monitor int, format string, quality int, err error) {
+	line, err := r.ReadString('\n')
+	if err != nil {
+		return 0, "", 0, err
+	}
+	fields := strings.Fields(strings.TrimSpace(line))
+	if len(fields) != 4 || fields[0] != "C" {
+		return 0, "", 0, errors.New("bad capture request")
+	}
+	monitor, err = strconv.Atoi(fields[1])
+	if err != nil {
+		return 0, "", 0, err
+	}
+	format = fields[2]
+	quality, err = strconv.Atoi(fields[3])
+	if err != nil {
+		return 0, "", 0, err
+	}
+	return monitor, format, quality, nil
+}
+
+func writeFrame(w io.Writer, img models.CapturedImage) error {
+	if len(img.Data) > config.MaxCaptureBytes {
+		return errors.New("frame too large")
+	}
+	if _, err := fmt.Fprintf(w, "O %s %d %d %d %d %d\n",
+		strings.ReplaceAll(img.ContentType, " ", ""),
+		img.Left, img.Top, img.Width, img.Height, len(img.Data)); err != nil {
+		return err
+	}
+	_, err := w.Write(img.Data)
+	return err
+}
+
+func writeErr(w io.Writer, msg string) error {
+	msg = strings.ReplaceAll(strings.TrimSpace(msg), "\n", " ")
+	if msg == "" {
+		msg = "capture failed"
+	}
+	_, err := fmt.Fprintf(w, "E %s\n", msg)
+	return err
+}
+
+func readResponse(r *bufio.Reader) (models.CapturedImage, error) {
+	line, err := r.ReadString('\n')
+	if err != nil {
+		return models.CapturedImage{}, err
+	}
+	line = strings.TrimRight(line, "\r\n")
+	if strings.HasPrefix(line, "E ") {
+		return models.CapturedImage{}, errors.New(strings.TrimSpace(line[2:]))
+	}
+	fields := strings.Fields(line)
+	if len(fields) != 7 || fields[0] != "O" {
+		return models.CapturedImage{}, errors.New("bad capture response")
+	}
+	left, _ := strconv.Atoi(fields[2])
+	top, _ := strconv.Atoi(fields[3])
+	width, _ := strconv.Atoi(fields[4])
+	height, _ := strconv.Atoi(fields[5])
+	n, err := strconv.Atoi(fields[6])
+	if err != nil || n < 0 || n > config.MaxCaptureBytes {
+		return models.CapturedImage{}, errors.New("bad capture size")
+	}
+	data := make([]byte, n)
+	if _, err := io.ReadFull(r, data); err != nil {
+		return models.CapturedImage{}, err
+	}
+	return models.CapturedImage{
+		ContentType: fields[1],
+		Data:        data,
+		Left:        left,
+		Top:         top,
+		Width:       width,
+		Height:      height,
+	}, nil
+}