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/lint-invalid/hue-budget.tex

@@ -0,0 +1,18 @@
+\documentclass{standalone}
+\usepackage[en]{supertensor}
+\stsetrole{a}{stTeal}
+\stsetrole{b}{stOrange}
+\stsetrole{c}{stCoral}
+\stsetrole{d}{stViolet}
+\stsetrole{e}{stInk}
+\stdim{x}{2}
+\begin{document}\begin{tikzpicture}
+  \strow{row}{x}
+    \stface[role=a]{A}{}{x}{x}
+    \stface[role=b]{B}{}{x}{x}
+    \stface[role=c]{C}{}{x}{x}
+    \stface[role=d]{D}{}{x}{x}
+    \stface[role=e]{E}{}{x}{x}
+  \strowend
+  \sttopformula{F}{$A+B+C+D+E$}
+\end{tikzpicture}\end{document}