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/gstack-links.test.sh

@@ -98,4 +98,12 @@ check T4-nothing-created "$([ -e "$DST4" ] && echo present || echo absent)" \
 check T4-warns "$(printf '%s' "$out4" | grep -qi 'refusing' \
   && echo yes || echo no)" yes
 
+# ── T4b: dst under src with a missing parent — refused, nothing created ──
+DST4B="$SRC/missing/x"
+link_gstack_helpers "$SRC" "$DST4B" >/dev/null 2>&1
+rc4b=$?
+check T4b-rc "$rc4b" 1
+check T4b-nothing-created \
+  "$([ -e "$SRC/missing" ] && echo present || echo absent)" absent
+
 printf 'PASS=%s FAIL=%s\n' "$pass" "$fail"; [ "$fail" -eq 0 ]