Assembly stylesheet maintenance

[Rob Beezer]

Just completed a pass through the pretext-assembly.xsl stylesheet, fixing up
some technical debt and some other items.

* There has been a fair amount of mayhem with ids. The "visible-id" template
has been retired in favor of the "unique-id" template, which has long been
functionally equivalent. There are @original-id and @assembly-id attributes
that are only rarely used. The latter has a "safety" template for access - I
mightn have seen some bare use in Oscar's texstyle work (now prefixed, see next
item).

* Every additional attribute created by the assembly process is now in the "pi"
private namespace, to prevent abuse. ;-) Should not have a big impact, but
developers will need to know about it.

A few more announcements coming in other places.

https://github.com/PreTeXtBook/pretext/pull/3080

Rob