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/absolute.tex

@@ -0,0 +1,7 @@
+\documentclass{standalone}
+\usepackage[en]{supertensor}
+\stdim{d}{3}
+\begin{document}\begin{tikzpicture}
+  \stface{A}{(0,0)}{d}{d}
+  \sttopformula{F}{$A$}
+\end{tikzpicture}\end{document}

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}

tests/lint-invalid/callout-duplicate.tex

@@ -0,0 +1,13 @@
+% Two asides on one band: a dashboard, not a figure. The second card belongs
+% in \stmeaningbox.
+\documentclass{standalone}
+\usepackage[en]{supertensor}
+\stdim{T}{4}
+\begin{document}\begin{tikzpicture}
+  \strow{R1}{T}
+    \stface{A}{}{T}{T}
+  \strowend
+  \stcallout{N1}{3cm}{R1}{first}{one aside per band}
+  \stcallout{N2}{3cm}{R1}{second}{this one has nowhere to go}
+  \sttopformula{F}{$A$}
+\end{tikzpicture}\end{document}

tests/lint-invalid/formula-order.tex

@@ -0,0 +1,9 @@
+\documentclass{standalone}
+\usepackage[en]{supertensor}
+\stdim{d}{3}
+\begin{document}\begin{tikzpicture}
+  \sttopformula{F}{$A$}
+  \strow{row}{d}
+    \stface{A}{}{d}{d}
+  \strowend
+\end{tikzpicture}\end{document}

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}

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}

tests/lint-invalid/raw-rectangle.tex

@@ -0,0 +1,6 @@
+\documentclass{standalone}
+\usepackage[en]{supertensor}
+\begin{document}\begin{tikzpicture}
+  \fill (0,0) rectangle (2,2);
+  \sttopformula{F}{$A$}
+\end{tikzpicture}\end{document}