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/smoke.tex

@@ -1,3 +1,5 @@
+% supertensor-lint: allow-absolute, allow-missing-formula
+% Legacy absolute-placement smoke test; flow layout is covered by flow.tex.
 \documentclass[border=8pt]{standalone}
 \usepackage[cjk]{supertensor}