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

analysis/API_REFERENCE.md

@@ -90,11 +90,13 @@ ws://<telescope_ip>:9900/?client_id=<client_id>
 
 ### 2.3 RTSP preview
 ```
-rtsp://<telescope_ip>/<stream_selector>/stream0
+rtsp://<telescope_ip>:554/<channel>/stream0
 ```
-- `<stream_selector>` comes from `StreamTypeAnn` (`com.convergence.dwarflab.data.bean.camera`).
+- `<channel>`: `ch0` = Tele camera, `ch1` = Wide camera
+- **IMPORTANT**: The camera MUST be opened via WebSocket (`CMD_CAMERA_TELE_OPEN_CAMERA` 10000 or `CMD_CAMERA_WIDE_OPEN_CAMERA` 12000) before the RTSP stream becomes available. Without opening the camera, the RTSP server accepts connections but never sends frames.
 - Player options force `rtsp_transport = tcp` (see `RtspPlayerView.java:195`).
-- Two cameras exist: **Tele** (main/long-focus) and **Wide** (wide-angle/guide).
+- Codec: MJPEG over RTSP (not H.264).
+- Port 8092 (MJPEG HTTP `/mainstream` and `/secondstream`) exists but is **inactive** on the DWARF Mini — the Mini uses RTSP exclusively.
 
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