After mlr-org/mlr3pipelines#882 is in mlr3pipelines, write a short subsection about it in the book:

Recommended location

Chapter “Sequential Pipelines” (sec-pipelines)
→ immediately after the section “Graph: Networks of PipeOps” and before “Sequential Learner-Pipelines.”
Add a new second-level heading, e.g.

## Convenience helper: `preproc()`

Section size –
The proposed position allows a short, self-contained sub-chapter (~½–1 page) with:

• signature and purpose;
• training vs. prediction examples on a Task and a data.table;
• warning about by-reference modification and the optional state;
• a pipeable example chaining several preproc() calls.