Pen Protocol

Nobody has claimed this yet.

There exists in fontTools the [pen protocol](https://fonttools.readthedocs.io/en/latest/pens/basePen.html), which acts as a mediator between things using their own outline drawing methods. The [scope of the API](https://github.com/fonttools/fonttools/discussions/2165) is to standardise the way outlines and components are drawn, without providing channels for metadata like glyph width.

What's interesting is that the protocol is not only used for mapping outline data from one internal representation to another, but also for e.g. filtering. It is a sort-of-pipeline for outline and component processing, available in a point-based and PostScript-like drawing model. Examples:

- Adapting a point-based drawing model to a PostScript-like drawing model: https://github.com/fonttools/fonttools/blob/4fb22ee/Lib/fontTools/pens/pointPen.py#L147
- Reversing a contour, e.g. for after you converted cubic to quadratics for a TTF: https://github.com/fonttools/fonttools/blob/4fb22ee/Lib/fontTools/pens/reverseContourPen.py#L9
- Recording the pen movement, e.g. for later replay into a different pen or for testing (with components optionally decomposed): https://github.com/fonttools/fonttools/blob/4fb22ee/Lib/fontTools/pens/recordingPen.py
- Testing whether a point falls inside the "black" area of a glyph: https://github.com/fonttools/fonttools/blob/4fb22ee/Lib/fontTools/pens/pointInsidePen.py#L13
- Converting cubic outlines to quadratic TTF-ready ones: https://github.com/fonttools/fonttools/blob/4fb22ee/Lib/fontTools/pens/cu2quPen.py
- Compute a hash for an outline for checking if some metadata is out of date: https://github.com/fonttools/fonttools/blob/4fb22ee/Lib/fontTools/pens/hashPointPen.py

This allows e.g. ufoLib2 and glyphsLib to implement just a `drawPoints` method on their glyph objects and use the existing pens to transport or transform the data without reimplementing the functionality at the source or destination, e.g. for compilation inside ufo2ft.

Start with the linked fontTools basePen API and the referenced examples in pointPen.py, reverseContourPen.py, recordingPen.py, pointInsidePen.py, cu2quPen.py, and hashPointPen.py. The issue does not identify norad files, tests, or a concrete scope; completion would require an agreed Rust protocol design and defined validation criteria.

Written by the indexing model from the issue text.