Function fetch 

pub async fn fetch<L: FrameLink>(
    device: &mut UlcpDevice<L>,
    keys: &[u32],
    batched: bool,
) -> Result<PropSet>

Read keys in as few exchanges as the device allows.

One CMD_PROP_MULTI_GET, continued where a reply ran out of room. A device without CAP_CMD_MULTI never learned the command, so it gets the same questions one at a time — the report is identical either way, and only the cost differs.

A key the device refuses lands in the set as a refusal rather than ending the read: assembling a report is exactly the case where one unimplemented property must not cost the other eleven.