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/screenshot/screenshot.go

@@ -7,6 +7,7 @@ import (
 	"image/jpeg"
 	"image/png"
 	"runtime"
+	"sync"
 	"syscall"
 	"unsafe"
 
@@ -157,19 +158,30 @@ func attachInputDesktop() error {
 	return nil
 }
 
+var (
+	enumMu  sync.Mutex
+	enumBuf []rect
+	enumCB  = syscall.NewCallback(enumMonitor)
+)
+
+func enumMonitor(_ uintptr, _ uintptr, monitorRect uintptr, _ uintptr) uintptr {
+	if monitorRect != 0 {
+		enumBuf = append(enumBuf, *(*rect)(unsafe.Pointer(monitorRect)))
+	}
+	return 1
+}
+
 func enumerateMonitors() ([]rect, error) {
-	var monitors []rect
-	callback := syscall.NewCallback(func(_ uintptr, _ uintptr, monitorRect uintptr, _ uintptr) uintptr {
-		if monitorRect != 0 {
-			monitors = append(monitors, *(*rect)(unsafe.Pointer(monitorRect)))
-		}
-		return 1
-	})
-	ok, _, err := procEnumDisplayMonitors.Call(0, 0, callback, 0)
+	enumMu.Lock()
+	defer enumMu.Unlock()
+	enumBuf = enumBuf[:0]
+	ok, _, err := procEnumDisplayMonitors.Call(0, 0, enumCB, 0)
 	if ok == 0 {
 		return nil, err
 	}
-	return monitors, nil
+	out := make([]rect, len(enumBuf))
+	copy(out, enumBuf)
+	return out, nil
 }
 
 func captureRect(r rect) (*image.RGBA, error) {