Add source linter, negative test fixtures, and fallback guidance

- scripts/lint.py: reject raw rectangles, absolute coordinates, hue-budget
  and callout/group/formula-order violations at the source level
- tests/invalid/ + tests/lint-invalid/: negative fixtures proving the
  package and linter reject bad input; test.sh now runs both directions
- references/fallback.md: degraded path when no LaTeX is available
- tests/group-callout.tex: exercise \stgroup and \stcallout
- agents/openai.yaml: agent config
- Docs and .sty updated to match

tests/invalid/role-redefinition.tex

@@ -0,0 +1,8 @@
+\documentclass[border=2pt]{standalone}
+\usepackage[en]{supertensor}
+\stsetrole{x}{stTeal}
+\stsetrole{x}{stCoral}
+\stdim{d}{3}
+\begin{document}\begin{tikzpicture}
+  \stface[role=x]{A}{(0,0)}{d}{d}
+\end{tikzpicture}\end{document}