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

references/semantics.md

@@ -65,3 +65,9 @@ One tensor role keeps one hue for the whole figure — that is what `\stsetrole`
 gathered, resharded or regrouped view of the same data keeps the *same* role color; a new
 hue means a new object. Derived tensors may reuse their parent's family rather than
 spending a hue (`V → O → Y` in the MHA example are all violet).
+
+A `\stgroup` outline follows the same rule, because a group *is* a regrouped view: give it
+the role of the objects it wraps — the three-sheet `q` stack and the outline that names it
+as one composite are the same object, and a new hue there would claim a new tensor exists.
+Only when the members genuinely differ in role does the group take `role=neutral`; that is
+also the honest signal that the box is naming an arrangement rather than an object.