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

SKILL.md

@@ -52,7 +52,7 @@ shape or convention you inferred.
 | data movement, collectives, non-linear ops | arrows and nodes | `\stlink`, `\stcomm` |
 | a split along the contracted axis | a vertical pair inside one band slot | `\stcol` … `\stcolend` |
 | adjacent objects that are one composite (heads of `q`, shards of `W`) | an outline in their own role hue | `\stgroup` … `\stgroupend` |
-| an aside about a whole band | a side card off its right edge, one per band | `\stcallout` |
+| an aside about a whole band | a side card off its right edge, one per figure | `\stcallout` |
 
 Combine grammars only when each one adds information. Known zeros stay unfilled; masks,
 diagonals, sparsity and partitions must encode their exact structure.