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/checklist.md

@@ -1,7 +1,8 @@
 # Pre-delivery checklist
 
-A clean `build.sh` proves only that TeX was happy. Nothing below is checked by the compiler.
-Work through it against the rendered PNG. Any mandatory violation means redraw, not patch.
+`build.sh` already checks the source rules, package invariants, missing glyphs and text-box
+overflow. The math, semantics and rendered relationships below still require inspection.
+Any mandatory violation means redraw, not patch.
 
 ## 1. Math (before looking at the picture)
 
@@ -30,18 +31,20 @@ Open the PNG at 100 %. Most of the first three items are automatic under the flo
 they still need looking at, because the cursor only guarantees that objects do not *push*
 into each other, not that the figure reads correctly.
 
-- [ ] No absolute coordinate that a `\strow`/`\stcol` could have expressed. Every remaining
-      hand-placed node is wrapped in `\sttrack`.
-- [ ] `\sttopformula` called after the bands, so the formula is centered on the real figure.
+- [ ] Every lint exemption in the source is genuinely required and explained; deliverables
+      normally have none.
 - [ ] No forbidden intersection, tangency, clipping or occlusion — including stack offset
       sheets, brackets, arrow labels and the meaning box.
 - [ ] Every connector's white label underlay covers only its own connector.
 - [ ] No connector crosses a box that is not its endpoint.
 - [ ] Symbols and shapes sit on two flat lanes per row; stage headings share a left rail.
+- [ ] Every `\stgroup` outline names a composite the computation actually has, is captioned,
+      and carries its members' role hue (`neutral` only for genuinely mixed members).
+- [ ] Every `\stcallout` is an aside about its whole band, not a step: one per band, no
+      taller than the band, and nothing in it that belongs in `\stmeaningbox`.
 - [ ] Top zone compact (≤2 formula lines, no shape underbraces).
 - [ ] Bottom box: one column, ≤3 rows, no overflow, no shrunken type.
-- [ ] Signature outside the box, one line, names what the figure actually shows, not clipped
-      and not visually dominant.
+- [ ] If requested, signature is outside the box, one line, accurate, unclipped and subdued.
 - [ ] Structural support exact: known zeros unfilled, masks and diagonals exactly right.
 
 ## 5. Thumbnail audit