Harden \stgroup and tighten the callout budget (review follow-up)

- Bracket ink is part of the fit: \st@facebody drops -inkw/-inke extreme
  coordinates and \stface/\ststack register them with the enclosing
  group/col/row fit, so a group outline can no longer be crossed by a
  member's bracket arms
- \stlink inside \stgroup or \stcol is now a package error: sub-flow
  members never terminate a pending connector, so the arrow was dropped
  silently while the label still rendered
- \stgroup requires role= (explicit role=neutral for mixed groups) and
  must bind at least two members or one \stcol partition; a lone stack
  or face inside a group is a dirty-build warning
- lint: default budget is one \stcallout per figure; the
  allow-multiple-callouts directive relaxes it to one per band
- build.sh: clean-build hint no longer names hue budget (lint owns it)
- tests/group-callout.tex reworked: multi-member group with a bracketed
  member as a regression probe, single callout; new negative fixtures
  group-link, group-norole, group-single, callout-budget
- api.md, checklist.md, style.md, layout.md, SKILL.md updated to match

tests/lint-invalid/callout-budget.tex

@@ -0,0 +1,17 @@
+% One card per band, but a card on EVERY band, is still a dashboard. The
+% default budget is one callout per figure; this must fail lint without the
+% allow-multiple-callouts exemption.
+\documentclass{standalone}
+\usepackage[en]{supertensor}
+\stdim{T}{4}
+\begin{document}\begin{tikzpicture}
+  \strow{R1}{T}
+    \stface{A}{}{T}{T}
+  \strowend
+  \stcallout{N1}{3cm}{R1}{first}{one per figure}
+  \strow{R2}{T}
+    \stface{B}{}{T}{T}
+  \strowend
+  \stcallout{N2}{3cm}{R2}{second}{this one belongs in the meaning box}
+  \sttopformula{F}{$A,B$}
+\end{tikzpicture}\end{document}