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

references/checklist.md

@@ -38,10 +38,11 @@ into each other, not that the figure reads correctly.
 - [ ] 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`.
+- [ ] Every `\stgroup` outline names a composite the computation actually has, binds at
+      least two members or a `\stcol` partition, is captioned, and carries its members'
+      role hue (`neutral` only, and explicitly, for genuinely mixed members).
+- [ ] At most one `\stcallout` in the figure (exemption comment if more), an aside about
+      its whole band, no taller than it, 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.
 - [ ] If requested, signature is outside the box, one line, accurate, unclipped and subdued.