> context --make does a --generate so you can skip that and the luatools 
> run is not needed either (selfupdate is seldom needed too)

So 'context --make' is all one needs?  I'll update the wiki.

> we get a .5pt height difference somewhere (nothing else different in a 
> trace) ... smells like a luatex bug (testing that now)

That is my suspicion as well.  But I couldn't reproduce it with plain
luatex.  Though maybe I needed to use equation numbers, however that is
done in plain tex (the context run doesn't show the problem without the
\placeformula).

I suspect some internal state is getting slightly corrupted and getting
progressively worse with the 30,000 repetitions.

I've been finding many unexplained 0.5pt or so differences, which may be
related to the underbrace problem.  The context, so to speak, is that I
am putting in the last edits to the book, after having gone through and
checked every page many times.  So I want to ensure that none of the
small changes has a big effect, but I don't want to check every page
again (or I'll vomit).

Thus, I run my compare-pdfs.sh script on the previous and the latest
pdf. It renders each page to a B/W png (using mupdf) and then compares
corresponding pages using 'compare' (one of the ImageMagick tools).  Any
differing pixels are in red, and the shared pixels are in very light
gray.  Then all the pages with enough differences are viewed with 'feh',
in order of most-to-least differences.  And I check that nothing major
has changed.

What I keep finding is that pages long after where I make a small change
somehow get changed, with roughly one-pixel shifts to parts of the page.
I'm attaching an example pixel diff.  It shows changes on p.72 of the
mss.  However, the only change to the source code was on p.58 (and was
designed not to change any page breaks, which it didn't).  That same
page (p.72) often gets similar changes with all kinds of small changes.

[Note to me: In case I need to recreate the source for debugging
purposes, it's the change from rev 41ff54 to 2467d5]

In case the script is useful to others (earlier versions are in the
contexttest repository somewhere), I am attaching it.  I use it as
follows with a bash command line.  (The awk in the pipeline makes sure
that only pages with some difference make it to the viewer.)

DPI=72 compare-pdfs.sh old.pdf new.pdf | tee book-print.compare72dpi | awk '{if (($3 > 0.02)) print;}' | sort -nr -k3 | awk '{print $1}' | xargs -r feh

-Sanjoy