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

examples/antipatterns.tex

@@ -1,6 +1,7 @@
 % Anti-pattern gallery -- four ways to draw a figure that compiles cleanly and
 % still teaches the reader something false. Left of each pair is wrong.
 %   ../scripts/build.sh antipatterns.tex
+% supertensor-lint: allow-absolute, allow-missing-formula
 \documentclass[border=10pt]{standalone}
 \usepackage[cjk]{supertensor}