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/callout-anchor.tex

@@ -0,0 +1,12 @@
+% A callout anchored to a face rather than to a band: the card lands beside a
+% single operand and reads as a step in the computation.
+\documentclass{standalone}
+\usepackage[en]{supertensor}
+\stdim{T}{4}
+\begin{document}\begin{tikzpicture}
+  \strow{R1}{T}
+    \stface{A}{}{T}{T}
+  \strowend
+  \stcallout{N1}{3cm}{A}{aside}{anchored to a face, not to the band}
+  \sttopformula{F}{$A$}
+\end{tikzpicture}\end{document}