Macro pipeline 

macro_rules! pipeline {
    ($cx:expr, $input:expr, $($stage:expr),+ $(,)?) => { ... };
}

Macro for creating a sequential pipeline (not yet implemented).

In the full implementation, this chains stages with cancellation checks. Use pipeline_n_outcomes or pipeline_with_final for now.

Example (API shape)

let result = pipeline!(cx, input,
    |cx, x| stage1(cx, x),
    |cx, x| stage2(cx, x),
    |cx, x| stage3(cx, x),
);