fix(preprints): render LaTeX math in automated-review reports

Reviewers reading LaTeX papers emit inline/display math (e.g.
$\Delta_{\cos} \in [0.02, 0.05]$); tex_escape was turning it into literal
source. Now math spans ($...$, $$...$$, \(..\), \[..\]) pass through verbatim
and compile as math (amsmath/amssymb added to the review preamble).

Robustness:
- Single-$ spans are guarded: a span containing a space-delimited prose word is
  kept literal, since reviewers sometimes quote a paper's UNBALANCED $ (which
  would otherwise pull a whole clause into math mode).
- build_preprint_pdfs compiles with math, and retries once with math-as-text if
  a stray/paper-defined macro breaks lualatex — one bad expression never costs
  the whole report (fired once across the 60-preprint rebuild; all 60 built).

Rebuilt all 60 preprints. Tests added for math passthrough + fallback.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: github-actions[bot]