I haven't looked at the output, but as long as it works with latexmk and looks as expected, the approach looks reasonable. Feel free to merge!

Sample index page:

Since this is replacing \placeholder/nc with semantic macros (without the "nc"), there are some slight whitespace changes in the main text.

Before:

After:

If this is undesirable, I fear I have to introduce "nc" variants of the macros. Please advise.

(Uh, same-as shouldn't be italics in the "after" picture. Will fix right now.)

Editorial teleconference: introduce \libconceptx to fix \cname use in ranges.tex. Otherwise good to merge.

This seems to have borked the bookmarks in the PDF:

Rats.

Is there any way to do different things in an \rSec vs. outside?

Repaired by #3286.

For the same_as example, the library concept index now also has an entry for same-as-impl, which is an exposition-only name only used in the implementation of same_as.
Was this intentional? :) Looks a bit weird, because why would a reader ever look up same-as-impl in the index?

Well, in general we also add exposition-only concepts to the index, because they might be used far away from their definition. That's certainly not the case for same-as-impl, but it feels slightly inconsistent to decide this on a case-by-case basis.

Ah fair enough, thanks!