Method next_frame

rustcv-backend-avf/src/stream.rs:227–263

等待并返回下一帧。 帧数据的生命周期绑定到 `self`（零拷贝借用语义）。

(&mut self)

    ///
    /// 帧数据的生命周期绑定到 `self`（零拷贝借用语义）。
    async fn next_frame(&mut self) -> CvResult<Frame<'_>> {
        use rustcv_core::error::CameraError;

        if !self.is_streaming {
            return Err(CameraError::Io(std::io::Error::other(
                "Stream not started — call start() first",
            )));
        }

        // 阻塞等待直到 delegate 推送一帧（或 channel 关闭）
        let frame_data = self.receiver.recv().await.ok_or_else(|| {
            CameraError::Io(std::io::Error::new(
                std::io::ErrorKind::BrokenPipe,
                "Capture channel closed",
            ))
        })?;

        self.current_frame = Some(frame_data);
        let f = self.current_frame.as_ref().unwrap();

        Ok(Frame {
            data: &f.data,
            width: f.width as u32,
            height: f.height as u32,
            // 使用 CVPixelBuffer 报告的真实步长，而非估算值
            stride: f.bytes_per_row,
            // 像素格式与上面请求的 BGRA 对应
            format: FourCC::new(b'B', b'G', b'R', b'A').into(),
            sequence: 0,
            timestamp: Timestamp {
                hw_raw_ns: f.timestamp_ns,
                system_synced: std::time::Duration::ZERO,
            },
            metadata: FrameMetadata::default(),
            backend_handle: &AVF_HANDLE,
        })
    }

    #[cfg(feature = "simulation")]
    async fn inject_frame(&mut self, _frame: Frame<'_>) -> CvResult<()> {