Function start_preview 

pub fn start_preview(
    app: AppHandle,
    state: State<'_, PreviewState>,
    pipeline_state: State<'_, CameraPipelineHandle>,
    camera_id: String,
) -> Result<(), CameraError>

Start the camera preview pipeline (M-CAM.2 / AUT-256).

Today: pure state-machine transition. M-CAM.3 (AUT-257) fills in the actual gst → wisp → readback → frame-channel pipeline behind this transition.

Errors

Returns CameraError when the state machine refuses (already running) or — once M-CAM.3 lands — when the gst pipeline fails to produce frames.