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/geometry.md

@@ -17,8 +17,12 @@ to exactly one physical edge length across the whole figure**. Equal shapes ther
 an equivalence class automatically: `Q` and `V` at `T×d_h` come out identical without you
 lining anything up by hand.
 
+Declaring the same axis twice with the same value is allowed. Redeclaring it with a
+different value emits a package warning, preserves the first value, and fails `build.sh`.
+
 Raw integers are accepted (`\stface{A}{(0,0)}{4}{4}`) but they opt out of the guarantee.
-Use them only for a face whose axis appears nowhere else.
+Use them only for a face whose axis appears nowhere else. The source linter rejects a
+repeated raw dimension greater than one; give repeated dimensions a symbolic name.
 
 ## The rules