Function forward_decoder 

pub fn forward_decoder(
    weights: &Sam3DecoderWeights,
    memory: &[f32],
    memory_pos: &[f32],
    memory_text: &[f32],
    text_attention_mask: &[u8],
    batch: usize,
    h: usize,
    w: usize,
    seq_len: usize,
    gguf_packed: Option<&GgufPackedParams>,
) -> Result<Sam3DecoderOutput, Error>

Run the full decoder (batch must be 1 for the boxRPB path).