engine: \global in front of a LaTeX length command

[tannevaled]

\global\setlength\xx{5pt} inside a group was a local assignment: it took
effect inside the group and was undone on the way out, in silence.
\global\xx=5pt and \global\advance\xx by4pt both worked, which is what kept
the hole out of sight.

Why TeX does not have it

tex.web §1211: a prefix applies to the assignment it finds after expanding
what follows — it takes "the next non-blank non-relax non-call token", so a
macro is expanded away. And \setlength is a macro there:

\def\setlength#1#2{#1 #2\relax}      % ltlength.dtx

so \global reaches the register assignment by itself. Here \setlength is a
primitive, which getXToken does not expand, and doGlobal's switch had no case
for it — the default branch put the token back and ran it locally.

The machinery was already in place: assignLength takes a global flag that
nothing ever passed as true. \setlength, \addtolength, \settowidth,
\settoheight and \settodepth now receive the prefix, so it works for a
\newlength register and for one of the engine's dimension parameters
(\textwidth is \let to \hsize — a different path, same rule).

Measured

	beamer (200 talks)	arXiv (200 papers)
documents lost	0	0
outputs changed	0	0
page count	1061 → 1061	3563 → 3563

Both corpora are inert, which is what a fix this narrow should look like: no
document in either set exercises \global\setlength in a way that reaches the
page. The behaviour is pinned by tests instead.

Both baselines were re-run with the current main binary rather than reused from
earlier today — main moved in between (#95 changed glyph widths by ~1.7%), and a
stale baseline would have credited this change with someone else's deltas.

A correction

One test case of mine was wrong before the engine was: I asserted \the\xx after
setting \textwidth. Removed — the parameter path has its own test.