A preset page loads only poster-sections.js, so a preset of a poster type
registered by another plugin's script (upg100-list) could only say No
posters. The generator now gives no presets to a type whose file is not on
the tree, and check:poster-sections fails on any such preset: 9 presets
remain (7 regions, all regions, configure).
The tree generator writes a page per preset but never deletes one, so the
dropped presets' pages (the 10 upg100 ones and countries-other) are removed,
and a new arm holds the built preset pages to exactly the presets.
ed0a1a57b1