Update PG submodule to 2.19

Bump PG from 4b3e564c (pre-2.19) to 88420fc9 (PG-2.19 tag).

Changes in PG 2.19 (20 commits):
- Switch default LaTeX variant to xelatex
- Update JSXGraph to latest version
- Add showInStatic option to RadioButtons and CheckboxList
- niceTables percentage width fix
- PTX output corrections (multiple choice structure, display math)
- MathObject reduction rule for m(nx)->(mn)x
- npm dependency update (braces 3.0.3)

Docker build and render confirmed working.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: drdrew42