// Appended only to a new source-bound copy of the frozen Queue and AsyncQueue.
// The actual owning-CPU / masked-task gate precedes the backend observation.
// Returns inert records; membership pins and mutable backend remain private.
impl<B: Backend, const N: usize> Queue<B, N> {
    pub fn os_k1100_observe_member(&self, handle: Handle) -> Result<Observation, Error> {
        self.execution()?;
        let index = self.index(handle)?;
        let member = self.entries[index].member.as_ref().ok_or(Error::Stale)?;
        self.backend.observe(member)
    }
}