fix(portability): make test green on macOS, GNU-only idioms replaced

The suite and seven libs assumed a GNU userland: `cmd | grep -q` under
pipefail (grep exits at the first match, the producer takes SIGPIPE,
rc 141 → 15 false "merged into" FAILs in gitflow-test), `sed -i` with no
suffix, `wc -l` padding compared as a string, `stat -c`, `touch -d`,
`realpath -m` (gstack-links refusal never fired), bare `timeout` off the
sanitized PATH (design gate READY BUT UNVERIFIED), `grep -oP` (update-all
emptied the plugin list). Thirteen suites were red on this machine.

Portable forms on the native userland of both OS: producer captured out
of the pipeline, `sed -i.bak` in tests and a temp-sibling `sed_profile`
on the user dotfile, `tr -d ' '`, python3 perms, `touch -t`, a
`realpath -m` emulation that refuses `..`, perl `alarm` for the 15 s
bound, `sed -n` token extraction. Regression tests: gstack-links T4b,
doctrine-citers Alphabet/Alpha flip, profile-set-managed T18 (failing CLI
no longer aborts `set`), new portability-census suite over the tracked
shell files. Linux run deferred (see TODO).

lib/tests/portability-census.test.sh

@@ -0,0 +1,60 @@
+#!/usr/bin/env bash
+# lib/tests/portability-census.test.sh — regression guard for GNU-only shell
+# idioms that break on macOS (BSD userland). Deterministic idioms only:
+#   sed -i with no suffix, stat -c, realpath -m, touch -d, grep -P,
+#   a bare /bin/grep (LRN-074: pin /usr/bin/grep).
+# Not covered on purpose: `cmd | grep -q` under pipefail (not decidable by
+# text; fixed structurally by taking the producer out of the pipe).
+# Usage: portability-census.test.sh [file…]
+#   no args: flip-test, then scan tracked *.sh + hooks/*
+#   args   : scan only those files; exit 2 on any hit
+set -u
+ROOT="$(cd "$(dirname "$0")/../.." && pwd)"
+
+RE="sed -i ['\"]|stat -c|realpath -m|touch -d|grep -[A-Za-z]*P([^A-Za-z]|$)"
+RE="$RE|(^|[^a-z])/bin/grep"
+
+# file:line (or file:*) exempt from the scan, each with its reason.
+ALLOW=(
+  "lib/tests/guard-bash.test.sh:221"  # deny fixture: GNU spelling
+  "lib/tests/guard-bash.test.sh:225"  # deny fixture: GNU spelling
+  "lib/tests/portability-census.test.sh:*"  # spells the idioms
+)
+
+_allowed() {
+  local entry
+  for entry in "${ALLOW[@]}"; do
+    entry="${entry%%  #*}"
+    [ "$entry" = "$1:$2" ] || [ "$entry" = "$1:*" ] && return 0
+  done
+  return 1
+}
+
+# scan <file>… → prints file:line:text per hit, rc 2 when any
+scan() {
+  local f rel line lno text hits=0
+  for f in "$@"; do
+    rel="${f#"$ROOT"/}"
+    while IFS= read -r line; do
+      lno="${line%%:*}"; text="${line#*:}"
+      case "$text" in [[:space:]]*"#"*|"#"*) continue ;; esac
+      _allowed "$rel" "$lno" && continue
+      printf 'GNU-ONLY: %s:%s:%s\n' "$rel" "$lno" "$text"; hits=$((hits+1))
+    done < <(grep -nE -- "$RE" "$f" 2>/dev/null)
+  done
+  [ "$hits" -eq 0 ] || return 2
+}
+
+if [ "$#" -gt 0 ]; then scan "$@"; exit $?; fi
+
+# flip-test: a planted GNU idiom must be caught before the real census runs
+PLANT="$(mktemp -d)" || exit 1; trap 'rm -rf "$PLANT"' EXIT
+printf '#!/usr/bin/env bash\nsed -i '"'"'s/a/b/'"'"' x\n' > "$PLANT/planted.sh"
+scan "$PLANT/planted.sh" >/dev/null; flip=$?
+if [ "$flip" -ne 2 ]; then echo "FAIL flip: plant not caught"; exit 1; fi
+
+mapfile -t FILES < <(cd "$ROOT" && git ls-files '*.sh' 'hooks/*' \
+  | sed "s#^#$ROOT/#")
+scan "${FILES[@]}"; rc=$?
+[ "$rc" -eq 0 ] && echo "PASS portability census (${#FILES[@]} files)"
+exit "$rc"