Function strip_srv_only_keys 

pub(crate) fn strip_srv_only_keys(
    meta: &mut HashMap<String, Value>,
) -> Vec<&'static str>

Remove srv-only keys from the meta a client gives a NEW block (pane.open over RPC or HTTP, object.CreateBlock). Removed rather than refused, so duplicating a pane still works: the copy is an ordinary pane, without an owner, a banner, or a claim to be a popup. srv’s own callers (OpenBrowser, the popup route) set these keys after this check, through open_pane directly. Returns the keys it removed.