Fix appendix references and portable LaTeX build

[HarounDominique]

Resolves #30.

Changes:

• use LaTeX tools from PATH instead of the obsolete /usr/texbin path; fail clearly when pdflatex is unavailable
• run cleanup portably with bash
• register subsection appendix references so \cref{app:route} resolves as an appendix

Validation: git diff --check and bash -n NASA-TM/build.sh pass. Full PDF build could not run because pdflatex is not installed in the environment.