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/group-unclosed.tex

@@ -0,0 +1,12 @@
+% An unclosed \stgroup swallows the rest of the band into its fit list; TeX
+% only reports the resulting empty row, which does not point at the cause.
+\documentclass{standalone}
+\usepackage[en]{supertensor}
+\stdim{T}{4}
+\begin{document}\begin{tikzpicture}
+  \strow{R1}{T}
+    \stgroup{G}
+      \stface{A}{}{T}{T}
+  \strowend
+  \sttopformula{F}{$A$}
+\end{tikzpicture}\end{document}