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/group-callout.tex

@@ -0,0 +1,78 @@
+% Regression test for \stgroup and \stcallout.
+%   - a group over one stack (the three sheets are one composite object),
+%   - a group over two adjacent shards (the pair is one partitioned parent),
+%   - a side card hanging off the right edge of each band.
+% No absolute coordinate appears anywhere in this figure.
+%   ../scripts/build.sh group-callout.tex
+\documentclass[border=10pt]{standalone}
+\usepackage[cjk]{supertensor}
+
+\stsetrole{act}{stTeal}
+\stsetrole{w}{stOrange}
+\stsetrole{q}{stTeal}
+\stsetrole{k}{stViolet}
+\stsetrole{v}{stCoral}
+
+\stdim{T}{7}
+\stdim{d}{5}
+\stdim{dh}{3}
+
+\begin{document}
+\begin{tikzpicture}
+
+\ststage{S1}{一、分组框：把已经排好的面命名为一个复合对象}
+\strow{R1}{T}
+  \stface[role=act, bracket=true]{X}{}{T}{d}
+  \stlink{l1}{三路投影}
+  \stgroup[role=q]{QG}
+    \ststack[role=q]{Q}{}{T}{dh}{3}
+  \stgroupend
+  \stgroup[role=k]{KG}
+    \ststack[role=k]{K}{}{T}{dh}{3}
+  \stgroupend
+  \stgroup[role=v]{VG}
+    \ststack[role=v]{V}{}{T}{dh}{3}
+  \stgroupend
+\strowend
+\stcaption{X}{$\mathbf X$}{$T\times d$}
+\stcaption{QG}{$\mathbf q$}{$h\times T\times d_h$}
+\stcaption{KG}{$\mathbf k$}{$h\times T\times d_h$}
+\stcaption{VG}{$\mathbf v$}{$h\times T\times d_h$}
+\stcallout{N1}{4.2cm}{R1}{分组框在说什么}%
+  {外框与里面的面同色，因为它圈的是同一个对象的多头视图，不是一个新张量。
+   框的内边距计入行的包围盒，所以下面的符号轨仍然从框底起算。}
+
+\ststage{S2}{二、分组框圈住两片相邻的分片，标注它们合起来是谁}
+\strow{R2}{T}
+  \stface[role=act]{H}{}{T}{d}
+  \stglyph{m2}{$\times$}
+  \stgroup[role=w, pad=2.2mm]{WG}
+    \stcol{Wc}{d}
+      \stface[role=w]{Wa}{}{dh}{T}
+      \stface[role=w, gap=0pt]{Wb}{}{2}{T}
+    \stcolend
+  \stgroupend
+  \stlink{l2}{}
+  \stface[role=q, bracket=true]{Z}{}{T}{T}
+\strowend
+\stcaption{H}{$\mathbf H$}{$T\times d$}
+\stcaption{WG}{$\mathbf W$}{$d\times T$}
+\stcaption{Z}{$\mathbf Z$}{$T\times T$}
+\stcallout{N2}{4.2cm}{R2}{一行一张卡}%
+  {同一个 band 上挂两张卡会被 lint 拒绝：并排的说明卡是仪表盘，不是图。}
+
+\sttopformula{F}{$\displaystyle \mathbf Z=\mathbf H\,\mathbf W,\qquad
+  \mathbf W=\begin{bmatrix}\mathbf W_a\\ \mathbf W_b\end{bmatrix}$}
+
+\stbbox{all}
+\stmeaningbox{mb}{17cm}{all}
+  {$T$ 序列长，$d$ 模型维，$d_h$ 头宽，$h$ 头数}
+  {}
+  {分组框是横向的子流：成员写在块里，所以它只能圈住相邻的对象；
+   两侧的内边距各占一份宽度，箭头也接在外框上而不是成员上——
+   否则连接线会穿过一个并非自己端点的框。
+   说明卡挂在 band 右侧，并把纵向游标压到自己底边以下，所以卡片过高只会撑开空白，
+   不会压住下一段。}
+
+\end{tikzpicture}
+\end{document}