More page builder and typesetter refactors?

[Omikhleia]

With #149 we'll have a really minimal page·ant page builder...

For my own reminder of things that could possibly be considered for inclusion in v3.x some later version on that basis...

• Some more controversial commits back-ported from experiment "priv-r3exp-pageant-refactor" (?)
• Some of the ideas from "priv-r4exp-pageant-requiem" (?) = big question mark here, last time I checked, I had broken footnotes in obscure ways 🤣 despite the rather cool code and added features. And for my own reminder also, some of the "refactor" and "requiem" stuff were not aligned in scope and aim... but where did I saved those notes about it? Too many branches, too many experiments...

Details to be described later...