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/antipatterns.md

@@ -54,7 +54,18 @@ See `style.md`.
 - **A formula line placed first.** It can only be centered on a figure whose width is not
   known yet, so it ends up visibly off-center. Call `\sttopformula` after the bands.
 - **A floating commentary card between two operands.** If it is not a real operation, it
-  belongs in the stage subtitle or the bottom box.
+  belongs in the stage subtitle, the bottom box, or a `\stcallout` beside the whole band.
+  A card anchored to a single face reads as a step in the computation, and two cards on one
+  band turn the figure into a dashboard; both are lint errors.
+- **A callout that should have been the meaning box.** If the card is taller than the band
+  it hangs off, it is not an aside — it is the **Mechanism** row, and leaving it as a card
+  only opens white space, since the callout pushes the vertical cursor below itself.
+- **A group border used as decoration.** `\stgroup` names its members as one composite
+  object; drawn around whatever happened to be adjacent, it invents a grouping the
+  computation does not have. If you cannot caption the outline, do not draw it.
+- **A group whose hue invents a new object.** The outline around the three `q` sheets is
+  still `q`. A fresh hue there claims a fourth tensor exists; use the members' role, or
+  `neutral` when the members really are of mixed roles. See `semantics.md`.
 - **A meaning box that repeats the shapes.** The shapes are already under every block. The
   box is for what the axes *mean* and what the operation *does*.
 - **Solving crowding by shrinking type.** The type hierarchy is a hard floor; move the