Add build/ as the LaTeX output directory ...

Compile with pdflatex -output-directory=build so generated files (.pdf,
.aux, .log, the epstopdf-converted logo, etc.) land in build/ instead of
cluttering the repo root. build/ is tracked as an empty folder via
build/.gitkeep so it exists right after a fresh clone; its generated
contents stay gitignored.

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