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/style.md

@@ -21,8 +21,9 @@ stages that preserve the primary path. No unrelated branches, no dashboard panel
 | operator | `\Large` | `st op` |
 | symbol | `\small` | `\stcaption` arg 2 |
 | shape | `\scriptsize`, muted | `\stcaption` arg 3 |
+| side card | `\scriptsize\bfseries` title, `\scriptsize` body | `\stcallout`, same tier as `st note` |
 | bottom prose | `\small` | `\stmeaningbox` |
-| signature | `\scriptsize`, low contrast | `\stsignature` |
+| optional signature | `\scriptsize`, low contrast | `\stsignature` |
 
 Never shrink below this to make something fit — see `layout.md`.
 
@@ -32,6 +33,10 @@ Separate tiles with a small white gutter and 0.5–1 pt corner rounding (`\st@ti
 this). Thin neutral brackets, `black!55`–`black!70` outer borders. No saturated
 tensor-colored outlines, no continuous spreadsheet grid.
 
+The one exception is `\stgroup`, whose outline is drawn at `role!65`: there the outline
+*is* the object being named, so the hue is doing semantic work rather than decorating a
+face that already has its own fill.
+
 **Encode support before magnitude.** Every known zero stays white/unfilled; every shown
 nonzero gets color. A diagonal matrix must read instantly as colored diagonal cells on a
 white field. `pattern=diag/band/lower/upper/causal/data` fill exactly the structural
@@ -69,16 +74,23 @@ Narrow bold label rail, left-aligned ragged-right `\small` content, 8–10 pt in
 Keep each row compact: prefer symbol semantics over numeric configuration. When it is too
 long, **remove content** — never add cards, columns or smaller type. Pass `{}` to omit a row.
 
-## Signature
+## Side cards
 
-One centered line below the box, outside it, low-contrast gray, `\scriptsize` or smaller:
-`\stsignature{<subject>}{<fit node>}` renders `<subject>@五道口纳什`. The subject must name
-what this figure actually visualizes. Keep it on one line, with a small but visible gap.
+`\stcallout` is the only sanctioned floating text card, and it is deliberately narrow in
+scope: one per band, hung off the right edge of a *finished* band, never between two
+operands. When the text outgrows the height of its band, it is not an aside — move it into
+the **Mechanism** row of `\stmeaningbox` instead of widening or shrinking the card.
+
+## Optional signature
+
+Add a centered line only when the user or house template requests it. Keep it below the
+box, outside it, low-contrast gray and on one line. `\stsignature{<subject>}{<fit node>}`
+renders only the subject. Do not append an author, handle or brand identity.
 
 ## Never
 
 Charts or metric insets not present in the primary formula. Decorative pills, banners,
-shadows, repeated separators, explanatory cards.
+shadows, repeated separators, explanatory cards other than one `\stcallout` per band.
 
 ## Reference image