engine: \@ifnextchar halves ## in the branch it picks, and package names expand

[tannevaled]

Two defects that turn out to be one story: no beamer theme had ever been loaded,
and every keyval value was silently lost.

1. \@ifnextchar must halve ## in the branch it picks

It does not insert the branch — it stores it first. ltdefns.dtx:

\long\def\@ifnextchar#1#2#3{\let\reserved@d=#1
  \def\reserved@a{#2}\def\reserved@b{#3}\futurelet\@let@token\@ifnch}

and \def\reserved@b{#3} scans the branch as a macro body, which halves ## a
second time (tex.web §479: a # followed by a # stores one #). This engine's
\@ifnextchar is a primitive that inserted the branch verbatim, skipping it.

keyval is built on exactly that:

\def\define@key#1#2{\@ifnextchar[{\KV@def{#1}{#2}}{\long\@namedef{KV@#1@#2}####1}}

The #### is halved once into \define@key's own body and a second time by
\@ifnextchar, leaving the one # that \def reads as the parameter text #1.
Without it the generated macro's parameter text was ##1 and bound nothing —
\setkeys{fam}{k=VAL} gave <<>>.

	\meaning\KV@fam@k
real LaTeX	\long macro:#1-><<#1>>
before	macro:##1-><<#1>>
after	macro:#1-><<#1>>

\@ifstar and \@ifnextbracket pick their branch the same way and are halved for
the same reason (\def\@ifstar#1{\@ifnextchar *{\@firstoftwo{#1}}}).

2. Package names expand

\usepackage, \documentclass and \LoadClass now expand their {name}.
Checked in one line: \def\nm{marker}\usepackage{zz\nm} loads zzmarker.sty in
real LaTeX and loaded nothing here. beamer builds every theme file name that way —
\beamer@calltheme does \usepackage[{#1}]{beamertheme\beamer@themename} — so
\usetheme{default} asked for a package literally named
beamertheme\beamer@themename.

Together

The frametitle template lives in beamerouterthemedefault.sty, so it finally
exists and a frame title renders; and because key values no longer leak, the
theme files stop contributing four pages of debris. On the minimal document that is
exactly the reference:

	before	after	reference
n frames	n pages	n pages	n
frame title	absent	present	present
body	present	present	present
page	364.2×273.15	364.2×273.15	364.195×273.147

Measured, including every apparent loss

200 beamer talks: 0 content lost, pages 1061 → 1095, vector paths
155 106 → 174 863. Eight documents stop producing a sized page; each was checked by
hand:

• four are corpus fragments with no \begin{document} — 0 pages is what real
  LaTeX gives them;
• one is a talk with no frame at all;
• one is a [fragile] frame whose verbatim body this engine cannot render
  either way — debris before, less debris after;
• one is unchanged when re-run cold;
• one (039a234b02c5…) trips the runaway guard on main too — the guard now
  fires sooner, so 40 KB of debris becomes none.

200 arXiv papers: the single apparent loss renders 2.46 MB in both binaries
when re-run cold — it had simply exceeded the sweep's 90 s cap.