engine: a table cell keeps its nested environments whole

[tannevaled]

collectTabularBody tracked brace depth — and a matrix is not braced here. It is the same fault #153 fixed in the maths alignment scanner, in the scanner next door:

\begin{tabular}{ c c }
 $\begin{matrix} a & b \end{matrix}$ & $\begin{bmatrix} c & d \end{bmatrix}$ \\

became four cells of fragments, and the maths layer refused each one:

texmath: unexpected "}"   "\begin {matrix} d_{\ell _2}}"

224 of the corpus's dropped formulas are that, all of them tables of matrices.

TeX protects a nested alignment with the brace \@array opens (latex.ltx:12101, reached through amsmath's \env@matrix), counted in align_state (tex.web §6738-6742, §7259-7264). This engine never expands \matrix, so the environment is counted instead, and \bgroup/\egroup count as the braces they are (§7492-7493).

A nested tabular was broken the same way and is fixed with it: its \end closed the outer table, so everything after it was swallowed. Its test fails on main too.

Measured — 200 arXiv papers, 200 beamer talks

15 documents change and 7 repaginate, for +5094 glyphs:

	avant	après	tectonic
	5	14	14
	41	35	34
	29	30	29
	33	32	34

The page error against the 157 tectonic renders falls 670 → 656. beamer does not move by one glyph.