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/design-tool-gate.test.sh

@@ -18,7 +18,7 @@ bad() { echo "FAIL $1 — $2"; FAIL=$((FAIL + 1)); }
 # own probes must never resolve a REAL 21st — else CLI_ABSENT_10 (and every
 # other case) would silently exercise this machine's CLI instead of the stub.
 if PATH=/usr/bin:/bin command -v 21st >/dev/null 2>&1 \
-   || [ -e /usr/local/bin/21st ]; then
+   || [ -e /usr/local/bin/21st ] || [ -e /opt/homebrew/bin/21st ]; then
   echo "FAIL precondition: system-wide 21st present," \
        "CLI_ABSENT case not hermetic"
   FAIL=$((FAIL + 1))