[chiconws]

Summary

Moves the H.264 WebRTC signaling and the camera snapshot from the Home Assistant integration into the client, so any caller can open a WebRTC session or grab a still image without knowing how the two hardware families differ. Adds a streaming guide to docs/.

Changes

• open_webrtc_signaling() picks the signaling endpoint for the detected hardware and returns a nanokvm.webrtc.WebRTCSignaling session on its own WebSocket. One offer goes in, and one answer and trickled candidates come out, on either family.
• Pro offer split and answer merge. The Pro negotiates video and audio as two peer connections. The session splits the caller's offer per media kind, merges the answers back into one, holds early candidates until the merged answer, and maps candidates both ways. It also sends the Pro's application heartbeat, because the Pro ignores WebSocket pings. split_pro_webrtc_offer() and merge_pro_webrtc_answers() are public for callers that run their own signaling.
• NanoKVMVideoModeError is raised from the iteration when the Pro reports video status -4, which means another video mode took the encoder.
• get_snapshot(timeout=20.0) (non-Pro) returns the first raw JPEG of the MJPEG stream. The Pro refuses it with NanoKVMNotSupportedError before any request, because opening MJPEG there stops WebRTC (Reading one frame from /api/stream/mjpeg freezes active H.264/H.265 streams sipeed/NanoKVM-Pro#157).
• Sync client. open_webrtc_signaling() joins open_ws() as async-only, because its WebSocket belongs to the client's event loop. get_snapshot() works as a blocking call.
• Docs. docs/streaming.md covers snapshots, MJPEG and the WebRTC session step by step, including the Pro's single encoder. docs/api.md keeps the method table and links to it.

Notes

• The non-Pro firmware sends an ice-servers event (its configured STUN/TURN) on connect, before the offer, since 2.4.2. The session doesn't expose it yet. It only matters when TURN is configured on the device.
• On the non-Pro, MJPEG and H.264 don't stop each other, but H.264 fell to about 55% of its frame rate while MJPEG was open (PCIe 2.5.2, H.264 direct).

Behaviour changes

None. All methods are additions.

Testing

504 unit tests pass, 29 of them new. Against devices:

• NanoKVM PCIe 2.5.2: get_snapshot() returned a complete 48 KB JPEG in 0.28 s.
• NanoKVM Pro 1.2.15: get_snapshot() was refused without a request.
• The integration's 653 tests pass with this branch in place of the released client, with its camera on open_webrtc_signaling().