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/flow.tex

@@ -58,6 +58,9 @@
 \stcaption{Wc}{$\mathbf W^{(r)}$}{$d\times T$}
 \stcaption{Z}{$\mathbf Z$}{$T\times T$}
 
+\sttopformula{F}{$\displaystyle
+  \operatorname{flow}(X,W)\;:\;\text{对象按自身边界框依次占位}$}
+
 \stbbox{all}
 \stmeaningbox{mb}{15cm}{all}
   {$T$ 序列长，$d$ 模型维，$r$ 低秩维}