docs: add streaming simplification rollout and validation artifacts

docs/STREAMING_SIMPLIFICATION_ROLLOUT_AND_ROLLBACK.md

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+# Streaming Simplification Rollout And Rollback
+
+## Rollout intent
+
+`SIMPLE_STREAMING` enables the new WebRTC-only control path:
+- direct `stream:requested` fan-out to the camera
+- no media-provider session requirement for request/accept/end
+- no `stream:frame` relay in the backend gateway
+- web client uses only WebRTC offer/answer/candidate signaling
+
+## Migration notes
+
+1. Native mobile is not yet on the WebRTC path.
+2. `MobileApp/package.json` does not include a native WebRTC dependency such as `react-native-webrtc`.
+3. `MobileApp/README.md` documents that the mobile app still relies on legacy `stream:frame` relay.
+4. Because of this, `SIMPLE_STREAMING` defaults to `false`.
+5. Web and backend are prepared for the simplified path now.
+
+## Safe enablement sequence
+
+1. Deploy backend and web changes with `SIMPLE_STREAMING=false`.
+2. Validate legacy mobile behavior still works.
+3. Validate web camera/client behavior with `SIMPLE_STREAMING=true` in a controlled environment.
+4. Add native mobile WebRTC support before changing the default flag value.
+
+## Runtime checks when enabling
+
+1. Request a stream from a web client to a web camera.
+2. Confirm the camera receives `stream:requested`.
+3. Confirm `stream:started` is emitted after accept.
+4. Confirm WebRTC offer/answer/candidate exchange succeeds.
+5. Confirm `stream:ended` tears down both sides cleanly.
+6. Confirm no `stream:frame` messages appear in gateway logs.
+
+## Rollback checklist
+
+1. Set `SIMPLE_STREAMING=false`.
+2. Restart backend instances.
+3. Verify `/streams/request` creates `start_stream` commands again.
+4. Verify mobile frame-relay streaming resumes.
+5. Re-run web and mobile smoke checks on the legacy path.
+
+## Known blocker
+
+Native mobile parity is blocked until a supported RN WebRTC stack is added and wired into `MobileApp/src/app-context.tsx`.