Port volume split point insertion from Pipeline 1

## Background

The `dtbook-to-latex` script (PR #137) supports multi-volume output: the XSL processes `class="volume-split-point"` markers in the DTBook to generate per-volume title pages and numbering. However, the step that *inserts* those markers has not been ported yet.

In Pipeline 1 this is done by the `ch_sbs_dtbookInsertVolumeSplitPoints` transformer, which adds split point markers to the DTBook based on e.g. chapter boundaries or page count before passing it to `dtbook2latex`.

## What needs to be done

- Port `ch_sbs_dtbookInsertVolumeSplitPoints` (and possibly `se_tpb_dtbSplitterMerger`) as a separate Pipeline 2 module
- Expose a combined `dtbook-to-latex-multivolume` script that chains split-point insertion and `dtbook-to-latex` for the common use case

## Design note

The two steps should remain separate modules — deciding *where* to split (page count, chapter boundaries) is independent of *how to render* (LaTeX). The combined script gives users a single entry point without mixing the implementation concerns.

Start by inspecting the existing dtbook-to-latex script and the Pipeline 1 transformers ch_sbs_dtbookInsertVolumeSplitPoints and possibly se_tpb_dtbSplitterMerger. The work is done when split-point insertion is available as a separate Pipeline 2 module and dtbook-to-latex-multivolume provides the combined entry point without merging the implementation concerns.

Written by the indexing model from the issue text.