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

scripts/build.sh

@@ -1,5 +1,5 @@
 #!/usr/bin/env bash
-# Compile a supertensor figure and export every delivery artifact.
+# Lint, compile, and export every delivery artifact for a supertensor figure.
 #
 #   ./scripts/build.sh figure.tex [outdir]
 #
@@ -23,6 +23,11 @@ OUT="${2:-$SRCDIR/build}"
 ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
 mkdir -p "$OUT"
 
+if [[ "${ST_SKIP_LINT:-0}" != "1" ]]; then
+  echo "==> lint $BASE"
+  python3 "$ROOT/scripts/lint.py" "$SRC"
+fi
+
 echo "==> xelatex $BASE"
 # supertensor.sty lives in assets/; keep it off the user's texmf tree.
 TEXINPUTS="$ROOT/assets:$SRCDIR:" \