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

scripts/build.sh

@@ -74,6 +74,6 @@ if [[ $status -ne 0 ]]; then
   echo "==> BUILD DIRTY: fix the warnings above before delivering." >&2
 else
   echo "==> clean. Now do the visual audit (references/checklist.md) --"
-  echo "    a clean build says nothing about collisions or hue budget."
+  echo "    a clean build says nothing about collisions or thumbnail legibility."
 fi
 exit $status