Add RTSP preview capture — working with both cameras

Major breakthrough: RTSP preview fully operational on DWARF Mini.

Key discovery: RTSP (port 554) only works AFTER opening the camera via
WebSocket. Without CMD_CAMERA_OPEN (10000/12000), the RTSP server accepts
connections but never sends frames. This was the missing piece.

RTSP details:
- Tele: rtsp://<ip>:554/ch0/stream0
- Wide: rtsp://<ip>:554/ch1/stream0
- Codec: MJPEG over RTSP (not H.264)
- Transport: TCP (confirmed from RtspPlayerView.java ijkplayer options)
- Resolution: 1920x1080 both cameras

Port 8092 (MJPEG HTTP /mainstream, /secondstream) is inactive on the Mini.
The DWARF Mini uses RTSP exclusively, not the HTTP MJPEG fallback.

Added `dwarfctl preview grab [--cam tele|wide] [output.jpg]` that:
1. Opens the camera via WebSocket
2. Waits 2s for RTSP to become available
3. Captures one frame via ffmpeg

Updated API_REFERENCE.md with the corrected RTSP details and the camera-open
prerequisite.

💘 Generated with Crush

Assisted-by: Crush:glm-5.2

dwarfctl/cmd/dwarfctl/main.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"math"
 	"os"
+	"os/exec"
 	"strconv"
 	"strings"
 	"time"
@@ -44,6 +45,7 @@ func main() {
 		cmdMonitor(),
 		cmdInteractive(),
 		cmdPano(),
+		cmdPreview(),
 	)
 
 	rootCmd.MarkPersistentFlagRequired("ip")
@@ -316,9 +318,12 @@ func cmdCamera() *cobra.Command {
 			},
 		},
 	)
-	// Add --cam to all subcommands
+	// Add --cam to all subcommands, --raw to photo only
 	for _, sub := range c.Commands() {
 		sub.Flags().String("cam", "tele", "camera: tele|wide")
+		if sub.Name() == "photo" {
+			sub.Flags().Bool("raw", false, "capture RAW instead of JPEG")
+		}
 	}
 	return c
 }
@@ -662,6 +667,45 @@ func cmdPower() *cobra.Command {
 	return c
 }
 
+// --- preview ---
+
+func cmdPreview() *cobra.Command {
+	c := &cobra.Command{Use: "preview", Short: "RTSP preview frame capture"}
+	c.AddCommand(
+		&cobra.Command{
+			Use:   "grab [--cam tele|wide] [output.jpg]",
+			Short: "Capture one frame from RTSP (opens camera automatically)",
+			Args:  cobra.MaximumNArgs(1),
+			Run: func(cmd *cobra.Command, args []string) {
+				cam := mustCam(flagCam(cmd))
+				ch := "ch0"
+				if cam == api.CameraWide {
+					ch = "ch1"
+				}
+				out := "/tmp/preview.jpg"
+				if len(args) > 0 {
+					out = args[0]
+				}
+				scope, cleanup := dial()
+				must(scope.OpenCamera(cam))
+				cleanup()
+				time.Sleep(2 * time.Second)
+				url := fmt.Sprintf("rtsp://%s:554/%s/stream0", flagIP, ch)
+				ffmpegCmd := exec.Command("ffmpeg", "-rtsp_transport", "tcp",
+					"-i", url, "-frames:v", "1", "-q:v", "2", "-y", out)
+				if err := ffmpegCmd.Run(); err != nil {
+					die("ffmpeg: %v (is ffmpeg installed?)", err)
+				}
+				fmt.Printf("frame saved to %s\n", out)
+			},
+		},
+	)
+	for _, sub := range c.Commands() {
+		sub.Flags().String("cam", "tele", "camera: tele|wide")
+	}
+	return c
+}
+
 // --- panorama ---
 
 func cmdPano() *cobra.Command {